食べたい。 (tabetai) = I want to eat. [informal]
食べたいです。 (tabetai desu.) = I want to eat. [formal]
So, basically as Jericho Desu said, use です (desu) in formal situations.
Think of it as an -i adjective (I'm not sure if it's actually considered an adjective in Japanese, but I know it acts like one). When an -- adjective has です (desu), it's formal. When it's just the adjective, it's informal.
-tommy