In active voice there are two present tenses: simple present (I go, he goes, I work, I do) and present continuous, aka progressive present (I am going, he is going, I am working, I am doing).
You use present continuous when you want to say you are in the act of doing something (Just now, I am writing a letter) or if someone/something is in the act of doing something. (The man on the bench in the park is reading a book).
In all other cases, that is when you don't want to express the continuous idea (in the act of doing) you use simple present.
In passive voice you have two present tenses as well, simple present passive (something is done) and continuous present passive (something is being done). The use is the same as in active voice, stressing the idea of being in the act or not.
I would recommend you to get a grammar for beginners from a public library, or better two grammars. Working with grammar notes on the internet is awkward.