we usually buy the bases from sainsbury's - you get 2 8" plain bases for about £1.
and you spread tomato puree on, then grated cheese, then whatever topping you fancy.
you can't put too much cheese on, because it doesn't melt properly, and tastes icky.
I like to put spinach on under the cheese, sometimes: it sticks to the puree.
mushrooms, sweetcorn, peppers, courgettes, asparagus, whatever's in season.
you can also use different cheeses, too, so goats cheese, or feta etc.
and you could use pesto instead of tomato puree as your spread.