I need some tips on how to stop sleeping on my back now that im pregnant?

Pillows. You are right to be concerned. Sleeping flat on your back could potentially damage your developing baby. You should be able to avoid changing positions in your sleep by placing pillows beside or behind you so that you cannot roll flat on your back in your sleep. There are special body pillows available to help with this as well.
Start with pillows. You can stitch 1-2 tennis balls into pockets on the back of a t shirt and wear that at night - if the pillows don't work this will.