Well check behind the speaker grills for some speaker wiring. Ford likes to build things so that items that don't come with your truck (like rear speakers or map lights in the dome) have all the wiring ready to go.
Speakers: Polk Audio EX368 $62 /pair shipped
Head Unit: Alpine CDA-9827 $230 shipped
(includes install accessories)
MTX 5-channel amplifier: $275 shipped
(245 w x 1 & 50 w x 4)
I just kinda picked that amplifier as it is 5 channel, but the subwoofer is a bit tricky to recommend. Since you have a regular cab, it the enclosure is everything. Do you want the passenger to suffer a loss of 2-3 clicks of reclining? You can get an install kit for the amplifier locally, probably around 20-50 bucks. Walmart can provide you with decent speaker wire in 50 to 100 ft spools. So it all depends on how far you want to go. The above system would be pretty rockin, and would cost around $600-625 fully installed (not including a sub and enclosure)
You may want to start with the speakers, and perhaps a head unit. If you don't think you'll be getting an external amplifier for a while, then don't go cheap on the head unit. Get as much power and quality as you can the first time.
The above list is from brand names and models I trust, but that doesn't mean it's the best for your setup and budget. Hopefully that gets you in the right direction, though...