If you don't really know what to expect, then it is best to start slow and easy and work up to harder sessions. Not good to start with a pro dom who will thrash you to bits, even if you think that's what you want.
Many WGs will offer a femdom and spanking session even if they are not full-on pro doms. Look for one who describes her activities in the text, not just someone who adds it to their likes list. They should describe what they do/don't, and more importantly how they discuss limits and safewords, etc. beforehand. Decide what sort of session you want: roleplay? leather/uniforms? verbal and/or physical domination/abuse? Then pick a WG who has been generally well reviewed on here, and also will allow the dom session to drift into a normal sex session at the end, that way you should have a good time even if the dom bit is not all you expected.
Finally don't be afraid to discuss the session at the end (allow 5 minutes for this). A good SP will tell you what worked or didn't and how you might take it forward (even if they are touting for a repeat visit it's useful info.).