When you make the "sandwich" is the backing completely free of wrinkles? Do you have a large enough flat area to either tape it down (like a clean kitchen floor, for example) or clamp it to a large table top? Make sure you're backing is wrinkle free before placing your batting on top of that and then your top. Pin, pin, pin, and pin some more!! Start your quilting in the center of the quilt and work to the ends. As I'm quilting, I give the backing a slight "tug" as I'm going along, especially after I remove pins that are in my path. This smooths any wrinkles that might have been in the backing from pinning.
When you're done quilting, square up the quilt before binding.
Don't give up. Take it slow. We were all beginners once upon a time. It will get easier as you get more experience.
Hope this helps.