When you lean to one side while riding your longboard, you should turn in that direction. This is called Carving. You can carve on your heel edge or your toe edge, and the deeper you carve, the more extreme of a turn you will make.
Try carving gently down the slope that you've been practicing on, and get comfortable with it. Carving slows you down, so you may need to either give yourself a stronger push. Try moderating your speed with carving, and get used to it. Try actually turning completely to the side by cutting a deep, hard carve. This is another way to stop, and how you will turn corners while longboarding. You can also slide, but that's a more advanced longboarding move. For now, just stick to getting comfortable with easy, gentle carving.
While practicing carving, you should also notice that the more you crouch down, the faster you go. If you stand straight up, you should slow down. Understanding little things like this will help you become more comfortable and smooth.
As you get more comfortable longboarding, try to not watch your feet. Keep watching further ahead, down the hill, and let your body naturally adjust for what's going on. This will help you avoid bigger problems later.