Basic twitter info needed?

[Lacey E]

I just joined twitter and I haven't quite figured it out. How do you communicate with someone without replying to them. Like if you want to leave a comment or tell them something. The only way I've figured out is to reply to an update they have made. I've read the Twitter help page. I also don't know when to use the "RT". I hate feeling this inept! Please help me twitter experts!