1. To begin with, you shouldn't join Pinterest just because "everyone else is doing it." You can browse your friends' boards without making your own, so don't feel like you have to make an empty profile unless you start seeing pictures everywhere that you don't want to lose!
Because that is the ultimate point. To keep all the pretty things you see together in one unforgettable place. It has nothing to do with how many followers you have, or how often you are repinned. Those things are nice affirmations of your style-sense, but if you have 100 followers and five pins, you're missing the point.
2. There are three things you can do to someone else's pin (other than commenting or ignoring, which would make five...). You can report it, which you should do if it is inappropriate, the guidelines for which are outlined under Pin Etiquette, which you should read before you begin. Seriously. My mother's on Pinterest. Let's keep it clean.
Other than that, you can "like" it, or you can "repin" it. Liking a pin does more than just stamp the pin with an effervescent symbol of your approval. It stores the pin in one big folder of your very own that says "Likes." The pins you like do not show up in the feeds of pinners who follow you, so it's a wonderful way to quickly collect little things here and there that you don't want to forget (a recipe or how-to-clean-with-lemon-slices tutorial) but that you also don't necessarily want cluttering up your follower's home pages.
3. When pinning from a website other than Pinterest itself, try your darndest to pin from a solid original webpage. One that will not change its content. This is called a permalink. Be careful of this especially when you pin from Tumblr or other similar sites. I cannot tell how many times I have tried to locate the original source of a pin only to find myself trolling through dozens of pages on someone's Tumblr or blog. It's not just about convenience, though. It's about crediting your sources. A photographer should be findable from his/her photograph. A designer should be findable from his/her quirky logo. This isn't just a guideline, either. Pinterest asks you to do it, and plenty of artists, designers, photographers, companies in general, etc. are legitimately worried about the implications of Pinterest when the sources of things are not maintained. Honor the artist.
These are only three pinterules, but they should be all you need, at least as you get going. In the meantime, here are some other great pinners to follow:
*Note, graphic above came from here.