How do you tell a girl that you want her to be yours forever?

First, by letter her be her's. It's ok to refer to her as yours, but make sure she knows you see her as her own person, not "belonging" to you.

Second, listen, listen, listen. The more you truely listen to her, even if it's not something you're interested in, the more she will respect you, and the more she will ultimately love you.

Third, treat her well, but don't spoil her too much. Make sure she feels loved, not bought.