You can send private messages but I think you need to have posted at least 50 times to be able to do that. You can increase your posts quickly by playing the word games that are usually to be found in the unread posts section if you scroll through you should find one.
To quote someone look at the bottom of their post and you'll see a quote button. If you want to quote the whole post just start your post below the quote. If you just want to quote a section, delete the bits you don't want. Make sure you keep the bits that look like this =
I don't really know what the point of rep power is but someone gives it to you for a post they've liked. It's a private thing so no-one but you can see it, unlike when you're thanked for a post.
To give it click on the scales at the bottom left of their post and a box pops up with a space for you to comment. You can write what you like, if you want them to know who sent it, don't forget to add your name at the end, if you don't want them to know, just leave it off.
Under the rep power part on the left you get coloured bits depending how much rep power you have. Thats about all I know about it. I just gave you some so you can see what happens when you get it.