go down to a sporting goods store, and buy an adjustable dumbell set. it is the cheapest way to add the most exercises to what you can do with nothing. it will cost probably $30-50USD. You can buy extra plates also for like $2.
squats you need a squat rack and a barbell. the best exercise IMO. it makes your quads and hamstrings sore as hell, and you will feel the difference in a week.
a curl is just a basic bicep curl. put about 20lb on each dumbell, hold them by your sides, and raise them up. try not to move your elbows or use momentum. go slowly, never jerk them. this will make your biceps larger.
for skull crushers, you lay on your back on some sort of bench. If you used 20lb for curls, maybe try 15lb for this. put your arms straight up with the dumbells in your hands. bend them back slowly so that your elbows are still straight up, and your forearms are perpendicular with them. now, keeping your elbows as still as possible, extend your arms completely.raise your forearms up. this will make your triceps larger. I like these a lot, just like squats, because they hurt like hell afterwards.
You may find another site to tell you how to do these and other exercises in detail. they might even have .gif images so you can see what they look like. if something doesnt feel right, stop..........