I have got united healthcare thru medicare anyone know about it

I'm currently trying to figure out my Insurance ,I  have united health care thru medicare and they pay  80% of the cost my question is do I pay the 20% up front to the dr. for the remaining balance first before sugery or after I go to New beginning in owasso and just still confused on some of the stuff that needs to be done, do I pay the hospital upfront or what, I don't even know what the actual cost is for they haven't told me yet when do you find out the cost I've had most of my pre-op test so waiting to see the surgeon again to know how can I find out. pleazzzz help me if you know for less stress is best.