Can someone explain twitter to me?

[iHATEmYrightEYE]

Yes you can "tag" someone.
It's called "mentioning" them.
For example, if your twitter username is "iamfunny" and I want to "tag" you in something I'm saying, I'll say:

"me and @iamfunny just came back from the mall" (and you HAVE to put that @symbol without a space before their username to "tag" / "mention" someone"

Pretty much, re-tweeting something is kinda like "liking" something on Facebook. If someone tweets something funny, a quote you like, or just something that you like, you click 're-tweet' and it will re-tweet that tweet to your followers under THAT person's username (username of whoever originally tweeted what you like) along with something under it saying that it's YOU that re-tweeted it.

Yes you can private message people, it's called a DM or Direct Message or simply Message. That person you want to message has to be following you as well, (and you following them), otherwise celeberities would have 49305893053 messages from people that they aren't even following.

Your friends will follow you if you ask. If you have Facebook, post a status saying that you have a twitter and that they should follow you and you can even promise you'll follow them back. Also, I find if you follow celeberities, some random people will somehow end up following you. They're not stalkers or anything but if you want you can "block" them to make them stop following you if you're creeped out by it. I don't block people I don't know that live millions of miles away from me simply because it just looks good to have a bigger number of followers.

Hope I helped!