Will this lens fit my camera? - 1

Does this Sigma 18-200mm F3.5-6.3 DC fit a Nikon D3100? Please help, I'm really confused and I want a cheap zoom lens! And is this lens good? Do you know a better one?

Sigma is a third-party lens manufacturer (they also make cameras, but that's beyond the scope of this question) which makes lenses for several of the major-brand camera mounts.
Just make sure that it specifies that it is "for Nikon" bodies and you'll be just fine. There are many "better" zoom lenses available, but not for the price of the Sigma. I would just double-check Tamron and Tokina just to make sure that you aren't missing a better deal, but Sigma is normally in the lower price range for those lenses. Good lens though.

Get it for nikon and you have a cheap zoom that fits. It does autofocus but it's a long way from good. Nikon's 18-200 is the only good superzoom.