welcome to the wonderful world of car audio. hope you enjoy your stay, haha. it can get complicated. but here you go.
for the front, the 5 1/4's will bolt right into the stock locations. just make sure that the new speakers arent any deeper (like if they have a huge magnet) than the old ones or you will have problems. get a set of component (= separate mid and tweeter) 5 1/4's if you can, itll make the wiring a lot easier.
for the back, 6x9s will fit no problem; im running them myself. they have to be spaced up about 1 1/2", however. just cut some spacers out of MDF or whatever else strikes your fancy.
now; as to wiring. take off the door sill pieces (plastic, they break SUPER easy but are very difficult to take off) and remove the piece of trim behind/beside the drivers seatbelt. you can thread wires underneath the carpet here, next to the stock wiring bundle. id advise you to drill a hole in the metal behind the back seat, near the bottom, to route the wires through. put something around the hole, and wrap all the wires in a couple layers of electrical tape, so they dont chafe.
wattage - just like paul said, something with 50-100wRMS power handling should do just fine. however, speakers are rated in dB (sensitivity), not S/N. it isnt really something to be concerned about unless your seriously into car audio.
speakers by mtx, clarion or alpine should work alright. if your not looking for amazing; dont spend a fortune. even relatively cheap aftermarket speakers can sound better than the stock ones. (although the stock ones sound pretty dang good with a better deck.) look at this
link for some ideas.
need more info, feel free to PM.