The most import thing to remember when it comes to getting spanked is that you should not do anything you don't want to do. You have an absolute right to express what you want, to slow things down or to call a halt to the spanking at any time.
Spankings often include role play. Spanking roles often replay conventional power relationships: teacher/student, boss/employee, nanny/child or parent/child. If your fantasies revolve around particular roles, tell you disciplinarian. As long as they are not uncomfortable with a particular role, they can use that in a spanking session.
What should you wear for your spanking session? Some disciplinarian's may require their spankee's to be naked for their punishment. However, many spanking sessions will start with the spankee wearing at least panties.
The spankee should wear something that allows their bottom to be easily exposed. Short pleated skirts (the "schoolgirl outfit") are always a good choice. Or lose pants. Tight jeans or skirts can make a spanking a struggle.
People who like spanking also tend to be obsessed with panties. Girls have a vast range of options, but boys can also get sheer or semi-sheer string bikini panties for men.
If the spanking session starts over the panties then wearing thong panties might not be a good choice unless the spankee wants their spanking session to be bare bottom. Sheer or lace bikini panties with a full coverage back are a good choice since they are sexy and provide at least some protection. Cotton panties provide more protection, but your disciplinarian may decide to pull them down at the start of the spanking session for this reason (white cotton panties often go with school girl spanking scenes).
When you get spanked, you are submitting to your disciplinarian. You can emphasize this submission by offering your bottom for punishment. For example, if you are going to be whipped with a strap you can present your bottom for punishment by arching your back and pushing your bottom up. A girl spankee should take her spanking with her thighs spread, so her pussy is displayed for her disciplinarian.
Hard or punishment spanking sessions include a possible ass fucking. When your disciplinarian has finished spanking you, they will give you a punishment fucking (see anal sex). If you are going to be fucked in the ass, you should obediently reach back and spread your buttocks as wide as you can while you wait to be penetrated.