My Best tip is dont start cheap, i wasted so much money on cheap audio, its crap. you can get cheap sub and amp packs for as little as £60 now, but there not worht the bother, if you spend the right amount you can get some good stuff, look for names like Jl,Alpine,Pioneer etc etc, names like that you cant got wrong with, if you go into your nearest halfords you can get a pay monthly head unit from as little as £10 a month for 10 months, good luck.