You've tiled and grouted, but after some months you notice that your wall tiles are allowing for the passage of considerable moisture--right through the stone. Your task, then, is to seal these pesky, porous wall tiles.
Clean the tiled area. Standard tile cleaner and a wet rag will accomplish this. Make sure that you remove all dirt, dust or filament of any sort from the tiles you will be sealing.
Dry the tiles. It is absolutely essential that your tiles be completely dry before you apply any sort of sealant. Use a clean, dry rag to dry the tiled area, and allow the tiles to dry from the inside out overnight.
Put on solvent-resistant gloves. Good stone sealant can irritate or even harm your skin, so wearing gloves for protection is quite necessary.
Apply porous stone sealer to the tiled area. Use a tile sponge to perform this application. Water-based sealer is probably best, since the wax-based sealers tend to wear down after a while, necessitating frequent resealing. Porous rock sealer is available at any hardware store.
Wait until the porous rock sealer has dried completely; then apply a second coat. Repeat Steps 4 and 5 until you have applied four or five coats of porous rock sealer. (If you use wax-based sealer, one or two coats should be all you need.)