You could also practise playing songs standing up too. Because when we perform on stage we don't sit down to play the guitar. You should also find a teacher to help you improve faster than you would if you learnt on your own. Make sure he's good enough to help you too.

These links should help you on the subject of guitar teachers.

http://tomhess.net/Articles/ChoosingATeacher.aspx

http://tomhess.net/Articles/DoYouReallyNeedATeacher.aspx

And it's a good idea to write your own songs now also.

Hope it helps,
Emmanuel Chan