First you need the right frozen pizza to act as a base. Thick crust varieties will never turn out very good. So you need a thin crust. Don't bother getting anything with toppings, you can add your own and they will taste far better than what generally comes on a frozen pizza.
|Here is a great place to start.|
Now you have the base, it is time to build upon it. First we need more cheese. Mozzarella is nice. A little jack cheese is good too. A good way to save money is to use "string cheese". Take 2-4 string cheeses, slice/grate/pull them apart, put on the frozen pizza. Just having a bit more cheese will greatly improve your pizza.
Now is where you can add some toppings. You can get good meat balls fairly cheap. Or you could use some ham. Really this is all a matter of what you like. Virtually any meat is good. Mushrooms are nice. Just keep in mind that you want to cut whatever you are going to use to be fairly thin.
The last thing to keep in mind is that adding things to the pizza will increase the baking time. If it says 18 minutes, it will be more like 20. However many factors effect cooking time, and your mileage will vary. This will also be quite a bit cheaper than ordering a high end pizza, and be better than any cheap pizza you can order.