if you're dead set on going to an install place then do, but it's pretty easy and theyll charge you for doing basically nothing!!
i'm no expert, but your car already should have wires leading to the rear ready to just crimp/solder onto your new speakers, if you're just running them off the head unit it's as easy as plug n play!
my advice would be get your mates to put them in.
if you try the spacers option like usmair did, then you could take it to autobarn etc to have it installed... ive found that they always have a way of making stuff fit, eg. with spacers/extra brackets etc.
something to think about, but by all means have a go yourself first it can't hurt