SMF is a free program and not a trial thingy, so you don't need to pay anything D-chan. SMF does have a Charter Member membership program that you can pay for but it's voluntary. If you become a Charter Member you get access to support services and get all of the newest gadgets and goodies before all of the free users do. So there are some advantages to paying, but it's not required to use SMF.
I've been with SMF almost four years and haven't had to pay for anything.

The main SMF site has bucketloads of canned theme skins to shop for if you want to change the look of the forum around.
http://custom.simplemachines.org/themes/