Sewage backing up from the toilet into the shower is obviously a problem you must fix immediately. A backup of this nature may indicate a clog in the main sewer line that all of your plumbing drains into, so sinks and washers eventually will back up, too. Another possibility is that the clog is in a shared drain pipe connected to both the toilet and the shower drains. In this case, the problem will remain specific to your shower and toilet.
A local clog affects only the shower and the toilet. The blockage in the line is somewhere between where the two drain lines tie together and the main line.
Use a plunger to force the blockage loose, according to Hometime.com. If you have one, use a fluted plunger designed to seal off the toilet bowl. You may want to get a second standard plunger and have an assistant seal off the drain in the shower to increase the thrust action of the plunger.
Plunge the toilet up and down several times and then quickly break the seal by pulling the plunger up. You may need to do this several times to force the blockage through.
Use a closet auger if plunging does not work. Using this small version of a pipe snake is often enough to break up local clogs.
Gaining Access To The Main Line
If your clog is in the main sewer line, you must determine the best place to gain access to the pipe. Since it is possible that the equipment will pull out some unsanitary mess, it is preferable to start at the outside cleanout. If your home does not have an outdoor accessible cleanout, find a basement or floor cleanout within the house or else pull a toilet to gain access to the lines, according to 411plumb.com.
Rent, buy or borrow proper drain snaking equipment to clear out the blocked line. Cable length is important because if it is too short it will not reach the clog, and if you let out too much it may become tangled in the septic tank or city sewer system, according to 411Plumb.com.
You will need the snake itself, as well as gloves that will allow you to feed the cable into the pipe. "Ugly gloves" are coated with a hard plastic that allows the cable to turn as you push it in.
Once you have the gloves on and the equipment in place, cleaning out the clog is quite simple. Turn on the rotating snake and gently feed the cable into the access point in the pipe. Once the cable reaches the clog, you will feel the resistance. Allow the head of the snake to penetrate the clog and move past it, then stop the machine and reverse it, allowing the snake to make another pass. If you feel more resistance, repeat as many times as necessary. When the pipe feels clear, reverse it and feed the snake cable back onto the spool.
With the main drain line now clear, your toilet and shower should drain properly with no sewage backup. Many professional plumbers can accomplish this task and already have the necessary equipment if you do not want to do it yourself.