Should I tell him how I feel if he has a girlfriend?
The best way to tell him is face to face; however, with his girlfriend around, that might not always be ideal. So a text message or a message via Facebook can help you get what you want to tell him across. Be honest. When you are dealing with feelings, honesty really is the best policy.
Should I just tell her how I feel?
No need to be very formal. Tell her not only your feelings but how special you think she is, be specific, in both external and internal things, ok, maybe not very specific but things like her smile, that you think she is beautiful, and a kind and smart person, by instance, and even that you enjoy being with her.
Is it OK to tell someone how you feel?
One of the most important things you can do when you decide you want to tell him how you feel is to tell him as soon as possible. The longer you put off telling them how you feel, the worse it is going to feel and the harder it is going to be to do because you'll build it up in your mind as something it's simply not.