Just a few things... Give a brief description of what you're looking for from your prospective beta.
Say what fandom this is for, or if it's an original story.
Let the prospective beta readers know what type of story this will be (in general). For example, if it's an erotic story then you really should point out what they can expect to find, so if it contains slash or rape, for example, and the prospective beta isn't into either of those things, they'll know not to bother enquiring. On the other hand, if it's right up their alley then they'll be more likely to enquire with you further. The prospective beta reader needs to know if what you're writing is something that will interest them.
State whether you'd like to read examples of the prospective beta reader's works before you agree on anything. This is a good idea, since if a person is going to be critiquing your work, wouldn't you like to know whether they really do know what they're talking about?
Very simple, and up to you guys to flesh out from that.