There should be a valve right above your water heater. Turning off the valve can provide immediate relief as you examine potential solutions.
Most homes have a main-line cleanout located in the ground. If you know where yours is, you can remove the cap to release the water from your home. This will relieve the immediate threat of a sewage spill on the interior of your home.
Your sewer system needs to be cleaned to remove any obstructions that prevent normal flow of sewage from your home to the sewer.
After the clog is cleared, your toilet flush may need to be adjusted to ensure it is flushing at the proper levels to ensure flow of sewage. Poorly adjusted toilets may not carry sewage all the way to the city sewer. If this is the case paper and waste settling in the pipe will build over time until it seals the pipe and restricts proper flow.
Maintence requirements will depend on the reason for the clog.
Tree roots are typically a once-per-year cleaning. Cleaning the drain prunes the roots from the pipe, but like the branches above ground, they will continue to re-grow.
If the clog was caused by foreign objects such as paper towels, tampons, and “flushable” wipes, maintence is as simple as ensuring that these items are not flushed down the toilet.