If I had to guess, you line up too far. You're parallel with the runway, just at the far side or not over it. This is VERY typical of new flyers. The good news is that it is really easy to fix. Try this....

First, when you fly, make sure that your feet are always shoulder width apart and more importantly, pointed perpendicular to the runway. Make your downwind leg and your turn to the base leg as usual. Here is the trick. Without turning your body, just follow the plane by turning your head. Keep the plane on the base leg until your head has turned as far as you can without twisting your body. THEN make your turn to final.

I know. You are sure that your plane with fly right over your head or even behind you. It won't. Your head really can't turn that far. It's a great way to guage when to make your final turn. Try it a few times at altitude. Trust me. It works.

Have fun!