how do i know when to use manges, mangez, mangeons? Quelqu'un peut m'aider?
It depends on the subject (je, tu, il, nous, vous, ils in French; I, you he, we, you, they in Engish). Verbs in French have different endings depending on the subject. You can see it like the s that you have to add to the third person in English, but with specifics for each of the six. See the conjugation table that curlyeric posted. You'll love them when you progress in French, guaranteed !
The first block of conjugations is present indicative, which is what you are looking for.