how to make a new clean installation?

hi.. i have uploaded wordpress first time than i have installed it but there are some themes that dont work with it they all show mixed up they arent looking at all like in the theme demo.but now i want to reinstall a new fresh wordpress but i cant because it says that i have already installed it and for new instalation i need to clean old database table…but i did deleted previous wordpress and uploaded the new copy, how the [moronic explicative] do i clean up the old database table? do i have to make a new database from my host provider?