How can i know the notes that I'm singing?

You first have to learn to hear note by ear so that you can pick them out. To obtain this skill you must PRACTICE!

Alternately, if you are trying to compose/transcribe, you can use an instrument. Sing the note steady, hold it, focus mainly on it, and guess at the note and play it. If you find yourself drifting, then try to refocus your attention and try with the instrument again, hopefully with a narrower set of notes to try.