I need help Biking hills?

I am training for a 30 mile charity ride in september. I am using a mountain bike as a road bike - since I can't afford a new bike right now. I have been out and done 3 training rides in the last 10 days. I have had to walk some of the hills because they are too steep and I am out of shape. Any suggestions on how to ease my way into the hills or at least get in shape to do them. Thanks