The whammy bar...
Frankly, on that guitar, the whammy bar is practically useless. You screw it in, and it's not supposed to do anything on its own. It won't pull back, only push down. When you press it down (which may give you a good bit of resistance) it should slacken the strings, like a whammy bar is supposed to do, but I'll bet anything that when it comes back up, it'll be out of tune.
Yep, it's worth it to have a locking nut with a whammy bar, if the bar's gonna get any use. Well, you could put some graphite in the nut to lube the strings so it's not so bad, but I'm telling you, that kind of bar is trouble.
And, unless you've been playing for a while already, the whammy bar is likely to be nothing but a distraction. By the time I actually used a whammy bar in my day to day playing, I'd already been at it for ten years. Now I can barely live without it, but with a locking nut my guitar will stay in tune for months, even after the most vicious dive bombs. A Fender will not. Or a Squier, same thing, right?
The back plate that covers the springs in the back, where you put the strings in, can be a huge pain when you're trying to change the strings on that guitar. When you take a string off, you have to try to poke it through the hole (heh heh, that's what she said) until it pops through and you can take it out. Ugh, and if the string breaks near the bridge, it can be a nightmare trying to get it out. Jimi Hendrix took his back plate off and left it off, because of that and so he could adjust the springs when he needed to. On my guitar, I never put the back plate on in the first place.
You can still play heavy metal with light strings, but if you use a low tuning heavier ones help. Or if you're me, or someone like me, maybe you just like heavy strings better. I'm not trying to talk you out of it, but you're better off choosing strings because 1) you like the way they sound/feel better, or 2) because they have the right tension for the tuning you use.
Changing strings is easy to do. I could try to explain how here, but I won't for two reasons. First, a few people already did, and I'm not trying to beat a dead horse. Second, it's waaaaaaaay easier to explain (and for you to understand clear as day) if it's done in person. Find a guitarist, have 'em show you one, then do the rest yourself. Once you've changed a few, you'll get the hang of it.