The requirements for donating plasma are often similar to those for donating blood - no IV drug users (past or present), no homosexual men or women who have had sex with men who have had sex with men in the last [X] years, no one with piercings or tattoos in the last [X] years who cannot provide a receipt from a reputable parlor for the body mod, etc, etc. Be prepared to pony up the info on your first and subsequent visits.
The physical reaction to donating plasma is not generally as severe as for donating blood, as your red blood cells are returned to your body mixed with a saline solution which replaces the plasma. The process can be uncomfortable if the needle is badly placed, so make sure that you cannot feel the needle itself once you've been stuck. Eat a full meal ahead of time if possible, and try to eat red meat or other sources of iron in the days leading up to your donation time, especially if you're female, and more particularly if you are PMS or menstruating.
Donating plasma can result in a permanent injection scar; obtain a donor card from the center you visit so that you can provide it to anyone who might discriminate against you, thinking you are an IV drug user.
As far as the payment goes, you generally need to donate twice in order to give enough plasma to be medically useful. Many donation centers, as a result, will give you a small fee the first time and the larger fee every second time you go. They will also regulate how frequently you donate - anticipate it being every second week.