how to prove a language is decidable
By definition, a language is decidable if there exists a Turing machine that accepts it, that is, halts on all inputs, and answers "Yes" on words in the language, "No" on words not in the language. Therefore one way of showing that a language is decidable is by describing a Turing machine that accepts it.

There are many offline and online resources on Turing machines. For example, here are some introductory slides on Turing machines: part 1, part 2.