First, make sure you're in edit mode (press i). Then you can paste with Ctrl+Shift+V, if you're in a terminal emulator like gnome-terminal (or select "Paste" from the right-click menu).

You can also type :set paste in vim before you paste to disable automated indenting, etc. Then :set nopaste after you've pasted the content.

Also check this question on stackoverflow.com for more information.

If you want to copy/paste lines in vim (as opposed to pasting clipboard content), you'll want to check out the yank command. Here is a cheat sheet that might help.