I am new to this modification thing and i recently downloaded the litestep installer and when i try to change the theme it says it may not work if i download the things i need, how do i know what themes work and which ones dont?

download a theme, install it (using the theme installer, or extraxt the zips to litestep\themes), and see. Over 90% of OTS and OTS2 themes will work. Most that don't are the fault of the themer (theme creation tip: delete the entire modules directory and do a few test-runs before releasing).