Straight answers.
No pipe dreams.

Take the tank lid off and watch what happens after a flush. Most running toilets come down to a worn flapper, a chain that is too tight, or a fill valve set high enough that water spills into the overflow tube. Replace the tired part instead of bending and fiddling with everything in the tank.

Stop when the job involves gas, a main sewer line, hidden leaks, permits, or live 240-volt wiring. Call sooner if you cannot positively shut off the water or if opening the wall could turn a small problem into an expensive one. Knowing where to stop is part of doing the job right.

Watch for a water meter that moves when every fixture is off, a warm or damp patch on the floor, unexplained water bills, or the sound of water running inside the house. Shut off toilets, ice makers, and irrigation first so they do not fool the test. A leak-detection plumber can locate the problem before anyone starts breaking concrete.

Put a screw-on pressure gauge on a hose bib and get a number before buying parts. Low pressure can come from a partly closed valve, clogged aerator, failing regulator, or restricted supply; high pressure usually points to a regulator problem. Pressure above 80 psi is too high for a typical home plumbing system and needs attention.

A tank heater stores hot water and is usually simpler and cheaper to install. A tankless unit heats water as it flows, saves floor space, and can run longer, but it may require larger gas piping, new venting, or an electrical upgrade. Pick one based on the house and peak demand, not the word “endless” on the box.

Once a year is a useful starting point, especially with hard water, but follow the manufacturer’s instructions. If an old drain valve has never been touched, forcing it open can create a leak that will not close again. When the tank is badly scaled or the valve looks fragile, have a plumber assess it first.

A dry P-trap is common in a sink, shower, or floor drain that rarely gets used, so pour in water and see whether the odor stops. Also clean the sink overflow and any buildup around the stopper. If several drains smell or you hear gurgling, the venting or sewer line deserves a proper inspection.

I do not recommend them. They can burn skin, damage some piping, and leave caustic liquid sitting in the trap for the next person who opens it. Try a plunger, clean the stopper, or use the correct mechanical snake—and tell the plumber if chemicals are already in the drain.

Find the main shutoff now, before water is running across the floor; it is often near the water meter or where the service line enters the house. Turn a wheel valve clockwise, or rotate a lever valve one-quarter turn until it is perpendicular to the pipe. Label it, keep the path clear, and test it gently once or twice a year.

Before freezing weather, disconnect hoses, protect exposed piping, and know that the main shutoff works. Every season, look under sinks and around toilets and water heaters for moisture, exercise accessible shutoff valves gently, and clean faucet aerators. Before a long trip, deal with active leaks instead of hoping they take the week off.