Immediately.  Start the way you want to finish.  If they were Sellers you would have them sign a listing agreement so why should a Buyer be treated any differently?   You need to become proficient in explaining the Agreement and why it is in their best interest as well as your own.  If they don't want to sign, you didn't waste any of your time.  I've found the only reason the odd person doesn't sign is that they had no intention of being loyal to you in the first place.  I like to know up front!

Wow!  I can't imagine any buyer signing a buyer broker agreement on the first outing (unless it was limited to any properties you showed them - and required by your state law).  Certainly not in order to be sent current listing information.  You and the prospective buyers really need to know each other a bit.  You need to know how motivated they are.  They need to feel comfortable that you are experienced and know what you are doing.  And remember, that piece of paper is not what is going to earn their loyalty and trust.

Right out of the box. Get the housekeeping done, before wasting any further time with them., If they refuse, they are not serious.

I present the agency agreement and its benefits at the first meeting. I rarely have a problem with getting anyone to sign.

You have a lot of great responses already, but I wanted to add my two cents as well. If they are long distance and haven't even come to town yet, I'd definitely send them information and talk to them about the area without worrying about having them sign the agency agreement or disclosures or anything else. 

When they finally arrive for the first meeting, I'd have them meet me in the office and go over all the paperwork before heading out to see homes.

I'm going to be honest here. 99% of the time I don't have anyone sign an agreement.  Either they want to work with me or they don't. 

At yor first meeting

In North Carolina, you have to send them agency information at the point of first substantive discussion. As for when they sign an actual representation agreement, that is really up to you. I don't typically ask for them too early on. Often I don't have them signed until we are ready to make an offer.

You should explain agency relationships in your state at the first scheduled meeting. You need to reach a professional comfort level with them to obtain a buyer agency commitment.

If they really seem into wanting to work with you...get it now. If they are just shopping on the internet, then bring them along slowly. 

As soon as you can/as soon as you can meet w/ them. Otherwise, there is an excellent chance you are wasting your time.

What Susan Emo says.  Have a discussion - face to face or if not feasible, via GoToMeeting.com or the equivalent.  In Arizona review the Agency Disclosure, the Buyer Advisory and the Buyer Broker Agreement.  Let them know that the documents will be made ready for their electronic signature in DocuSign, and once signed, the MLS Portal (listings) will be set up for them.

Nattalie de la Mothe Agency should be discussed as soon as substantive information about real estate is communicated to the prospective buyer. This should include signing a BBA.

Nattalie I would get them to sign as soon as possible after they indicate they would like to partner with you in finding them  a property to purchase.  I wish you well.

The sooner the better. If it is a cold lead from internet~ they might be not loyal and work with multiple agents.........

I sit down with my clients prior to showing them homes and go over Office Agency and Buyer Exclusive Agency and the home buying process. I get it signed there.

First in-person meeting before showing property is ideal, but I'd send them an advanced sample copy to read before that meeting. Good for you for thinking of this. Most agents are whimps about this. Good luck!

Debbie Laity has a great answer to this too.

I don't use them. There are useful in certain situations but I have never needed one.

You are new. It may be a good learning experience for you to find and to show them a few homes. On a subsequent meeting you can ask them to sign the agency agreement. If they won't, just consider the time you spent with them as tuition paid and learn from it. At the very least you will become more confident.

Congratulations, Nattalie de la Mothe .  What William Feela and Women of Westchester Working Together said.  

The best time is to do it immediately, start the meeting with an office visit, get all of th paperwork done before starting the job.

Agency agreement

Proof of financing or funds

Expectations for agent and client

etc

Congratulation Nattalie de la Mothe ! First prospect call. I can feel the excitement and uncertainty.....be the real you.

Your great goal is to PROTECT the buyers who have hired you. This means you really need to make sure they understand a few things about the local real estate market place. There are things in place that can prove damaging or incredibly disappointing for your clients if you ALLOW them to walk into the trap.

So during your first meeting, you protect them by discussing these traps you want to protect them from and suggest they avoid.

Those traps are:

1. Flat fee brokers who submit a listing with buyer agent compensation of one whole dollar. 

2. Brokers who under-compensate the buyer agent well below accepted standards

3. Sellers who want to transfer costs traditionally paid by the seller onto the buyer.

In all of these scenarios, the BUYER will be required to pay the deferred expenses by reducing the amount of down-payment available, digging into the remodel money set aside for the move or find out they can not buy that house.

"It is my responsibility to protect you from theses predatory situations. That is why we are having THE TALK about compensation in addition to my other responsibilities to you. Please sign here so my broker knows I'm doing my job."

This solves a BIG BOAT LOAD of problems.

At your first face to face meeting.

Good morning Nattalie. If you are going to fully committed to them, sign an agreement would be a minimum requirement.

All the best with your client.

I will show a few properties like Fred says. Probe around see if they are ready. Preapproval letter is important. After 2nd showing if they can not produce it I will hold off showing and you can say something like your broker wants all clients to sign and need a preapproval letter...

I do not use the form. It shows we do not trust them.  We have many agents and many buyer prefer to strike a deal directly with listing agent. 

Congratulations!  Seems like everyone has a different approach. I have explained the benefits and asked for their signature after our first walk thru. I'm sure they will but No one has declined yet. 

I agree 100% with Fred Hafdelin  on this answer.

Debbie Laity  has my answer. we dont use them out here much. in some places they are common place. i personally have met folks we ARE not good for. i had two showings today..one of them i WOULD never build a home for. sorry, but for me i am married to them for a year. no way on gods green earth i would work with them. but i am 105 years old and i can smell trouble a mile away. as i have often repeated from an old mentor. some of the best sales you ever made were the ones you let go

When they come to start looking get it signed.  

Nattalie de la Mothe ..it is not so much when even though relevant but how to do it. Remember, people buy from people first and then the product

Nattalie de la Mothe may be I am going to say something different!

Build the trust, ask them that they will work with you only - and mention that you work with clients committed to you only!

Do they need to sign the agreement? Make sure you have the trust relationship - and not just 'contract' relationship.

Nattalie - I think you have received a lot of great advice, and as you can see, agents deal with this in a number of different ways. I would go along with Kat Palmiotti and try and get them to sign as soon as you can get them in the office. For now, send them the information they are looking for. 

I like Susan's advice.

Congratulations! I'd say ASAP.

At least have the conversation right away. Know exactly what sort of representation YOUR state offers (it varies, I foundf!) and what it means to both of you in terms of your responsibilities to each other; be prepared to explain that! Once they understand and agree that they want to work with you, have them sign.

In Colorado you are to do this at the first meeting.

Probably varies by state, but usually before you start having substantive discussions about properties, procedures, etc, which usually means after you meet with them, they interview you, and decide that they like you and want you to be their agent. At that point, there is no reason for them not to sign one. And the expiration date you put on the agreement is flexible. It can be for 1 day, 1 week, 3 months, etc. You can give them a copy of it at first meeting and ask them to review it, and just explain that if they decide to hire you, you need it signed, etc.

 Good Sunday morning Natalie. Congratulations on your first prospect call. If anything, reach out to them and find their level of interest.