How do I tell her I love her?

by saying it.
its the only way

This really depends on how long you have been in a relationship with her. Saying "I love you" too early on in a relationship can make things awkward and uncomfortable. Before doing anything, make sure you both are comfortable enough with your relationship.
As for actually saying it, you can do so many things. Be creative! I've heard stories from friends about the ways their boyfriends let them know how they feel. You can do anything, from writing a note and slipping it into something of hers (lunch, coat pocket), to taking her to a restaurant and telling her there.

Best thing to do my friend is wait for her to tell you that she loves you first otherwise you might scare her off.