Hello mate, as it only seems to be me and Octane2097 replying to most the threads on here i thought i would comment aswell,lol.
As Octane said try and match your sound system up, always seems to sound better (and well i think it looks better aswell,lol) get yourself a good headunit, look at the watts per channel on the head unit (normally on the front, will read something like 65w x4) This means (as far as im aware, please correct me if im wrong) that headunit will push out 65w per channel (up to 4speakers ie 2 front 2 rear - like i said not sure if thats 100% correct) The headunit is the base/source of your sound system so make sure thats a good quality.
Then looking into speakers, again try and match the make and get a name you have heard of a trust, like Octane said Song Xplode are good i had a set of 6x9's from the xplode range and they really kicked.
Personally im not into Subs and stuff, but again try match the kit up and get yourself a good sub-box or boot build and it will kick like a donkey.
6x9's on the parcel shelf arnt really a good idea inless you have a stealth shelf, without one your 6x9's are on show to the whole world. I have a good tutorial/ guide for making a stealth shelf if your intrested mate (or anyone who reads this) just send me a PM and i will pass it on.
Lastly is a very small point (but i think a very very important one) Keep your wiring neat and tidy. Try to keep your joins in wires neat by the use of a connector block or spade connector, dont just wrap 2wires together and put black tape round them. Seriously the amount of cars iv seen where the guy or girl has just done that, its shocking. I was in one car where i couldnt put my feet on one piece of the carpet because the wires were under it and if i did the rear speaker would cut out.
Thats about it really mate, theres a few other points about Amps and other pieces of equipment but once you have the usual bits in, front speakers and rear speakers (either in the normal place under the grills either side of your boot or 6x9's) then look into amps and subs, as you may not feel you need all that Jazz.
Hope this helps mate.
Keep us updated with pictures and progress, this forum is seriously lacking input from members!
(Maybe a note to Admin??)