you now they are following because it says "Follows You" near the username or where they live
You can protect your account by going to your settings and ticking the "protect my tweets" box.
you need to go on your friend tree in your room, and go to the right hand side at the bottom it will say 'add new friend' type your friends username in then click on add the send them a request, or...
Because people are so used to acting like they are that they find it difficult to change. However some people find it easy. :P hope this helped :) x
By Mentioning that person username as @username or by replying to one of his/her tweets.