Browse the Meat Birds Etc section of BYC for lots of threads on how to dispatch, clean and cook your roo, if you don't find another taker for him. To give you a general overview, though:
Personally I'd recommend using a hatchet - you can set two nails about an inch, inch-and-a-half apart in the top of whatever you're using as a chopping block, to put his head between to steady him and stretch the neck out. Swing with resolution; you might want to practice on a stick from a tree or something, first. Alternatively, you can hold his body/neck in one hand, grasp the head just behind the back of the skull, and pull out and bend up til you feel the spine separate.
Other methods, some of them good, exist, but those are probably your best bets for your first time. I would not recommend trying to swing him around 'like a lasso' by his head, as that may very well not break his neck, at least not anytime soon, and if it doesn't would be pretty hard on the chicken. I mean, if you've done it a bunch before and gotten reliably good at it, fine, but it's really not a good bet as your first try.
Skinning is easiest and fastest if you've ever skinned game etc before; otherwise it's probably a tossup between skinning vs scalding-and-plucking. There are some good online directions, step by step with photos, for dressing the carcass -- look at the sticky atop the threads in the Meat Birds Etc section. It is not difficult, and an interesting biology lesson.
For cooking a 4 month old cockerel, you can use any method you like if you don't demand melt-in-the-mouth tenderness; if you are afraid the meat will be too chewy for your liking, try marinating in buttermilk for 6-12 hrs before cooking and then use a slow moist braising method to cook, e.g. with some liquid (not buttermilk) in a covered casserole in a slow-to-moderate oven. It will probably be the best-tasting chicken you have ever had, much more flavorful than anything from the store. By all means make stock from the bones, THAT will be more flavorful than anything from the store TOO
Good luck,
Pat