Walls
In tub and shower areas avoid dark grout colors as shampoo will cause them to turn whitish. After showering, rinse off the walls with clear water. Soap scum and body oils will rinse of easily before they dry but will be hard to clean if they build up. Occasional cleaning with a neutral cleaner will keep things looking good with minimal effort. If soap and body oils do build up and regular cleaning does not remove it, products like Scrub Free will dissolve them for a one-time cleaning but regular use will severely damage grout and can also damage marble, limestone and travertine, especially if they are polished.
The best way to prevent mold and mildew is to use an exhaust fan during and after showers and keep shower doors open. With constant heat and moisture, mold will grow on anything, but good ventilation and a fairly simple cleaning schedule will usually keep it from being a problem. Sealing the grout will help keep mold out and occasional bleach- based cleaners can be used, but repeated use of bleach will also lead to grout damage over time. Using exhaust fans will also help prevent mold growing in other places, including your attic.