do NOT trust Home depot's cut !!!...
they use the BIG cutting machine to cut the 4 x 8....
and its not accurate.... you might ended up being off by 1/4 inch!!! (that's what I got by asking them to cut for me !!!...)

they only do straight cuts, and I don't think they'll do angle cuts....
well..... if you simply not feel comfortable cutting at all... then I guess only way is to get a gutted one...
but if you're just can't cut very straight, then I suggest you spend like $10 or so, and get a 4 ft metal ruler and a couple of cramps.... and use the ruler as a guide to the saw... then you'll be able to cut perfect straight cuts.....
(that's how I do it now...)
hope it helps....