Welcome, and congratulations on wanting to learn more about the geekier side of Linux and server administration. The Linux learning curve can be steep, but at least you only have to learn it once.
You can find a set of basic Linux commands
here. But before playing with them on a live server, let me make a couple suggestions:
1. Remember the old proverb: "He who hacks much at root will kill tree."
2. ALWAYS make a backup before editing critical files. This may mean backing up the file itself (the cp command is your friend), or it may even mean making a VZ backup if you're doing something really, really dangerous.
3. Buy a good book about Linux. There are tons of them out there. You are probably running centOS on your VPS, which is almost identical to RedHat; so you may want to buy a "Dummies" book for RedHat (or centOS, if one is available).
4. I strongly advise you to buy an old laptop on eBay. Anything with at least a 1 GHz CPU and at least 512 MB RAM oughtta do ya. Then download centOS or Fedora and install it on the laptop. (If you buy a book, there may be a CD included that you can install.) Then practice on your laptop before you start hacking your server.
You can also install Linux on an older PC that no longer runs Windows well, or even
build your own Linux box out of spare parts, if you want. Either way, I think it's good to practice on a machine that's not a live Web server until you're comfortable with Linux.
Linux is about as close to a miracle as there in is IT. That something that originated with a college kid's desire for a cheap Unix box should become such a stable, elegant platform as it is still amazes me.
Good luck,
Richard