If you're comparing things, it's called a simile. For similes, 'like' and 'as' are synonyms, so you can use either one. However, 'like' is by far the more common word, and it's easier to use. These days, 'as' sounds poetic or literary, so unless you're writing a poem or using the phrase 'as X as', just use 'like'. For example: He looked like he was about to be sick. He looked as if he was about to be sick. ('as' needs 'if' here, 'like' does not) Her eyes shone like a thousand stars. (bad poetry) Her eyes shone as bright as a thousand stars. (still bad) So, 'as' is more difficult to use, and it sounds poetic in many cases. There are times when 'as X as' is absolutely necessary, though, so learn how and when to use that expression. In other cases, I would say that you should stick with 'like'.

It's a big topic. Grammar books have pages dedicated to it. I would recommend taking a session with a teacher and/or studying the grammar books. If you just did the latter, you could write some sentences of your own containing "as" and "like" and post them on Italki for correction.