Wrist curls are a great forearm lift. I'd do low rep, high weight sets of 4x8 or 5x5 if you are trying to prepare for heavy deadlifts. http://www.passion4profession.net/en/forearm-exercises/
Another great one that I do often is behind the back wrist curls. normally do 4x12 of a decent weight so that Im struggling on rep 8 or 9.
If you have a lot of wrist flexion in your bicep curls that will also work your forearms a bit, but the two I listed above are good starting lifts.
2 ways and 2 different exercises to get better.
unhinging your hips towards the back and then towards the front. This is a movement similar to squats where you will bring your hips/ass backwards - then forwards - as you gain more hip flexibility and control you'l be able to bring those hips forward and backward with power and speed. This will be very useful for you to grind the crap out of your S.O. Requires strong Glutes (ass) and strong flexible hips.
abductor muscles to move your body up and down. http://www.passion4profession.net/muscles-exercises/adductor-exercises-l.jpg. Basically by making these muscles strong and some flexibility youl have the ability close your legs and open your legs by sliding your knees away and towards your body while on top. The stronger and more endurance these muscles have the faster and stronger youl be able to go up and down.
As you become more experienced and skilled you can combine both movements. Hope this helps
OK like every person who replied to this kept saying "traps" so I googled it and they seem to be back and shoulder muscles just in case anyone else was wondering
You won't see much gains until you start using free weights. Machines just aren't as effective.
There are plenty of online resources to help you with form, and you can ask trainers at your gym.
This website is pretty good as it has videos and 3d models; and teaches you about which muscles you are using along the way.
At 21 you are still at peak testosterone levels, just start using free-weights please.
I've never used a machine for lower back. This is a good exercise to start with http://www.passion4profession.net/en/lower-back-exercises/. I've always found using a ball very comfortable, although I'm sure other more experienced lifters have better advice.
Also, Deadlifts.
Ahh great, well about the skin, if you're still young and a dietist checks on your diet, you shouldn't worry. for working out I'd reccomend basic push ups on your knees as shown here: http://www.passion4profession.net/en/chest-exercises/Page2/
Also the one shown in the picture of this article (english isn't my native langugage so I don't know how you guys call this one):
http://www.passion4profession.net/en/chest-exercises/
And last but not least the dumbbell bench press, those last 2 are great just make sure that you do them properly and take care of yourself, I've seen people getting smacked in the mouth with the barbell/dumbbell because they wanted to go Rambo on the workout.
There are others but for starters go with these, they do wonders on women.
Good luck!