How to improve my Mk3 GTI audio.?

Im not very good with sound system, but i have stock everything except for the HU, which i got a sony but im probably gonna get a newer one. I was going ot get two 12" subs with a sub box and an amp from my friend so that slves the b problem. But idk wht to get for my speakers and if i even need tweeters. Anyone got any ideas? I also dont know what im going to do in terms of arranging all of this into my car.