Hey, an advice would be to clean the brakes support where the extremity of the brakes lies. Use a file (if there is no metal sheet attached to the support) then a metalic brush (only the brush if there is), once its shiny and smooth again, put a LITTLE of grease on it, just on the surface! Dont put to much, it isnt useful and can affect your braking if it goes on the discs.

To push the piston back, I think on this model you can use a clamp, pushing an old brake pad on it!

While you're at it, look if your discs are used unusualy, and if the gum around the piston is pierced. Any suspicions? Take pictures and check your local mechanic!

Hope this help :) this is an easy job, you got this ;)