Here in the US, if you are in an undergraduate program, you can not become an assistant professor.. You have to be in the Masters program which would be year 5,6, or 7.
If you are undergrad, in years 3 and 4, you MIGHT be able to be a "T.A. which stands for "teachers assistan" and help grade papers and whatever else the professor what's you to do, but you will never be left in charge of the class .Even then, they will choose someone on beginning Masters program for that probably year 5. Undergrads need to be studying to get their first degree, not working for a professor.
You can try for teacher assistant position by asking the professor of they need one.