Hi Marcelo. Commas are usually used to separate clauses or items in a list - basically anywhere you would naturally want to take a pause when speaking. One common mistake made by English learners is in using a comma to link two parts of a sentence together when a linking word or full stop is actually required. Hope this helps:)

Hard to help without more information. It'd be better if you wrote a longer entry and attempted to use punctuation to the best of your ability. Then you can get corrections and specific advice on what you're doing wrong.