How do I get the water out of the shower that has backed up in it? How do I unclog it? It is connected to the toilet because it looks like toilet water has backed up.

Remedying the 2nd question will also remedy the first question. All plumbing drains eventually to the same sewer line. The toilet is close to the shower so the backup shows there first. If you have a septic tank, it may be full & require "pumping out" by a commercial company. They will always tell you that you need new drain fields. Don't automatically believe them. If you have city sewer, then there is a clog. In either case, try drain cleaner chemicals such as Drano, ETC. in the shower drain. Do NOT use these cleaners in the toilet. In the case of septic tanks, roots may be growing inside the tank filling/clogging the tank. There are chemicals for these as well. If all this fails. Remove the shower drain cover & run a "snake" through the drain to clear the clog.