You may want to review GCDs of terms first.
Factoring a polynomial means finding things you can multiply together to give it. For example, you can factor x2 + 2x by saying that it's x * (x + 2). To get that, you just work out the GCD of all the terms of the polynomial. In this case, there were two terms, x2 and 2x, and their GCD is x. So that's one of the factors. You can divide by that to get the other factor, which in this case is x + 2. (This division will always come out evenly; if not then you made a mistake somewhere.) So you write the answer as x(x + 2).