You'll need one or two wrenches depending on your setup. Look at the terminal, it should have a nut on one side with the other side being either another nut, or a big square. If it's a big square, you'll only need one wrench. If it's another nut, grab two. You can use open end wrenches or something more universal like pliers or a small Crescent wrench. It's always best to use a properly sized tool if it's available, but since we're not race car mechanics, sometimes we just have to make it happen.
You should always remove the negative cable first. Place one wrench on the outside nut and (if needed) the other wrench on the other side. Turn the outside nut counterclockwise until you're looking at 1/16-inch or so of unscrewed space on the bolt. If you're lucky, a slight twist will pull it right off. If not, read on for removal tips.
If your terminal seems pretty stuck, take a flat head screwdriver and gently pry the terminal open by inserting it in the gap above the bolt. Don't try to pry the terminal upward off the post. You could pull the post out with dire consequences. If it's still stubborn, give it some serious twisting strength, even using some large pliers to give it a twist. This should loosen it up.
Now that you're looking at just the battery and terminals, you can clean them up.