It is very easy, just talk to them. I have a very good friend called Elaine, we talk one day, and did not speak for about 3 months. We would always wave and smile as we crossed paths. I did not have the balls to talk to her as she was the hottest thing at school! Well it was about a week before school ended and I managed to get the balls to talk to her. Now we are super cool pals! When I moved to the UK, she gave me a picture and on the back it said '.......I am pleased you got the balls to come and talk to me, as you are a really sweet guy......' I am also pleased I got the balls to talk to her!
Just remember you can not force anyone to be your friends, but a bit of style goes a long way!