Study different physics textbooks. Looking in various texts will offer problems and opportunities to practice equations. Choose question sets with solution keys so you can check your answers.
Search the Internet for online physics courses. Virtually every accredited university and college has a beginning physics classes. These courses often offer lecture notes online as well as practice problems. Even some high school AP physics courses are good sources for practice.
Use basic algebra to solve density equations. For many density equations, you will be given all you need to solve the problem at the outset. Practice solving for density first. Then try to find the mass, given the value for density and volume. For instance, water has a density of 1. If you were given a mass of 7 g, then the volume would be 7 x 1 or 7 ml.