When did you buy your MacBook? Or more importantly, which revision of the MacBook is it? Run System Profiler and look for the "MacBookX,Y" number and report it here.
If your MacBook originally shipped with Tiger, and if you never properly bought Leopard for it, you'll need to buy the "Mac Box Set" (the bundle of Mac OS X Snow Leopard with iLife and iWork, US$199 if I recall correctly) in order to have a proper license for Snow Leopard.
If your MacBook originally shipped with Leopard, or if you properly bought the Leopard upgrade (US$129) at some point, then any version of Snow Leopard will do. (US$29).
If your MacBook originally shipped with some version of Snow Leopard, then it can get very tricky. If your MacBook shipped with 10.6.0, 10.6.1, or 10.6.2, you can buy the 10.6.3 revision of the Snow Leopard DVD retail kit and it'll work (but note that the original 10.6.0 revision of the Snow Leopard DVD retail kit won't work because it was too early to contain the specific drivers and fixes for your later MacBook). If your MacBook shipped even with 10.6.3, it would've been a slightly later revision of 10.6.3 that contained drivers and fixes specific to your MacBook, so the exact revision of 10.6.3 in the retail kit wouldn't have had those drivers and fixes, so it wouldn't work on your MacBook. So if your MacBook came with 10.6.3 or later, you should contact AppleCare or visit a Genius Bar or Apple Authorize Reseller/Service Provider and see if they can make you a copy of the install DVDs specific for your model of MacBook.