Ride more. Long steady distance rides would be ideal, but if you have to use a trainer indoors, use it for sessions lasting about an hour, at the maximum level of effort that you can sustain for that time. You'll find out what that is by trial and error. And make sure that you use a gear that allows you to spin the pedals fairly fast rather than grinding along. It's better for your knees, gives you a better cardio workout and is a good habit to get into.

You'll see very rapid improvements. Most important, though is to have fun - don't turn cycling into a chore or you won't stick with it. And as others have said, a 15mph average for a beginner is quite a challenging target, so don't be too hard on yourself.