If you don't know much about BDSM, negotiating with a pro dom can be challenging. Here are some tips.
When you initially contact your pro dom, you'll be asked what you're looking for in a session. Be prepared to cite some specifics--nothing is more annoying than someone answering, "Uh, the usual stuff, I guess...." There is no "usual stuff." A session centering around bondage and foot worship is very different from a session about caning and electrical play. Likewise, don't say, "I just want to be dominated." There is more than one way to dominate someone. Don't say, "I will do anything you want, Mistress!" Believe me, I can easily think of things you would not want to do....
Be prepared to say something like, "Well, I think I'd like to be tied up. And I fantasize about spanking and having someone put women's underwear on me." Understand, I am not saying that these particular things are what you should ask for, but you should be able to offer at least a sentence or two about why you contacted the dom (or domme). Don't ask for sex, though--pro doms don't offer this.
The tone of this contact should be courteous and pleasant on both sides. Just asking about a pro dom's services does not constitute agreeing to submit to her. Either one of you attempting to act as if you already owe some type of obedience is presumptuous. It's reasonable to expect to be treated with civility, honesty, and discretion in this contact. Your pro dom should be clear about her fee and her availability. And you should feel that your stated limits (meaning: what you don't want to do) will be respected. If she responds to your limits by saying something like, "Real slaves do anything the Mistress wants!"--run away, quickly.