First you will need to clean the tile/grout. Use cleaners that do not contain acids of any sort (This includes vinegar). Acids "clean" by removing desirable material, i.e., layers of grout from tile joints and elements of certain ceramic glazes from the surface of the tiles. Not good at all. Dishwashing soap and a stiff brush should do the job. Now about the sealer, there are many brands of tile sealers, all of which fall into two general categories: mineral based and latex or acrylic based. Use the latex based products. They won’t stink up your house like the mineral based ones will. Sealing a shower is easy. Make sure the shower is completely dry. It usually takes about five days for the moisture to evaporate from behind the tiles. Check the directions on the container. Generally, you wipe the product on with a sponge or rag, allow it to soak in for a minute or two, and wipe the excess from the surface with dry rags or paper towels. To make sure you’ve done a thorough job, repeat the process the following day. Allow the shower to dry overnight before you use it. Do not seal the shower floor. It needs to breath, and the sealer will inhibit it from doing so.