honestly curious to know about all 3 scenarios
First, I should state that I am writing from the point of view of a lee, since I am about 90% lee.
1) With a friend about whom you have no romantic feelings:
Clearly state that the object of the session is for both of you to have fun. Tell the friend that he or she may stop at any time. Make sure that he or she knows what a slow word is and what a safe word is, and what two words you are using for those purposes. If you don't trust your friend enough to respect your safe word, then cancel the session. If you do, then proceed. Tell your friend in advance what kind of tickling you know you like, and what kinds you would like to try out and find out if you like them.
2) With a person who is already (or you hope will become) your significant other:
Besides the advice above in 1), also tell him or her that tickling is an erotic activity for you, and that you will likely become aroused during the session. Make sure that he or she is alright with that. Make clear, in the case that you are not already intimate with him or her, that they are not obliged to do anything about it during the session, that they are free to ignore your erection. In the case that you are already intimate with him or her, make clear that it is a very good way to get you aroused, but also that they are not obliged to gratify you on the spot.
3)With a professional Dom or Domme whom you will pay for the session:
Make sure that they allow their clients the use of a slow word and a safe word. If not, then do not book the session.
If they do, then describe what you know you like as well as what you are not sure you like but want to try. Make sure that they are willing to do the things in the first category. Don't be shy or embarrassed about asking for exactly what you want. A pro Dom or Domme has heard MUCH kinkier and wilder requests, I can assure you.
Have the agreed tribute ready in a plain unmarked envelope when you arrive for the session and give it to him or her immediately. Save extra money for a tip until after the session if you feel you got what you wanted.