how can I be nice to girls without them thinking I am flirting with them?

Flirting and being nice are two different things. Hopefully they kind of over lap each other every once in awhile but trust me.... Opening a door for me is being nice, you are not flirting with me. Helping a damsel pay the last dollar she was short at the check out is being nice, you are not flirting. If you are my husband and you pinch my butt you are flirting but if you are NOT my husband and you pinch my butt that is NOT nice and I will slap you upside your head! If you whistle at a girl you are flirting but what is nice is you going up to the girl and telling her she has beautiful eyes. Giving someone flowers is an example of the flirt and being nice over lapping.

Treat them like your friends.  I have plenty of guy-friends, and I do all the same things with them that I do with my girl-friends.  It helps if your group of friends are all friendly to girls too.  Just say hi.  Be outgoing.  When you see someone you know, give them a high-five, like you would with one of your own friends.  After a while, it gets easier, and you won't even worry about them thinking you flirt with them.