If your shower gets hotter when someone flushes the toilet, it's a good bet that your shower faucet is an older one. Contemporary faucets incorporate pressure-balancing valves to prevent this unpleasant and unwanted scorching sensation. You may be able to convert your existing faucet simply by replacing the cartridge, but if the faucet is an off-brand or two-handled one, you may have to replace it. That isn't a difficult job for a pro, and may be within your purview if you're handy -- but it's messy. You must access the plumbing behind the shower, which means making a hole in the wall.
Install Replacement Cartridge
Note the manufacturer and model of the faucet, if it's a single-handled one. Look up the model on the manufacturer's website or call a plumbing supply outlet and purchase a pressure-balancing replacement cartridge, if one is available.
Turn off the water to the shower and remove the handle with a screwdriver or hex wrench. Pull off the temperature limiter, if there is one, and release the cartridge by unscrewing the collar nut with adjustable pliers or pulling a retaining pin with needle-nose pliers.
Note the cartridge orientation so you can put the replacement in the same way. Pull out the cartridge -- if it's hard to pull, reattach the handle and use that for leverage. Insert the new cartridge, and reassemble the faucet.
Turn on the water and test the faucet, adjusting the temperature limiter as needed to get comfortably hot water.
Replace Shower Valve
Cut into the wall behind the shower with a drywall saw to expose the existing valve. If you can't get behind the shower, look for access to the pipes through a closet or a wall in the adjacent room.
Remove the handles and outer trim from the old faucet. Turn off the water to the shower and cut out the old valve with a hacksaw or mini pipe cutter. Depending on your new faucet, you may have to cut the pipes back or reroute them slightly. You may also have to adjust the framing so the new valve is the proper distance behind the wall when you attach it.
Prepare the valve by screwing the appropriate adapters to its ports with a wrench. Pre-solder short lengths of pipe to each adapter so you can make solder connections without overheating the valve and damaging any plastic parts inside it.
Attach the valve to the framing with screws and dry-fit the plumbing connections with the appropriate pipe and fittings. When everything fits, solder each joint with flux, lead-free solder and a propane torch.
Turn on the water and check for leaks before you cover or close up the wall.