How do I get electric suply to sports changing room on a field?

You start by contacting a licensed electrician to develop and plan the electrical requirements and wiring plan first.

Some things to consider, length of run from supply feed to building. Conduit or direct bury. Type and size of cable needed. Will this locker room require both 120 and 240 (water heater).

Size, the outside measurements will help determine the minimum number of lighting circuits. You can always put additional lights.

You also need to determine where the power company will want the meter if this is going to a Main Service Panel.

Of course the NEC Code required Grounding must be installed to code.

None of this is trivial and that is why I strongly suggest the use of a licensed electrician.