You can purchase a sealer at the Home Depot (DIY Store) or at a tile place. The Home Depot will definitely be cheaper. 511 Impregnator by Miracle Sealants was recommended to me by my plumber and can be purchased at limited tile stores. It is also quite pricey and you really don't need much. Also you need to buy a bottle with a brush top that screws on for sealing (sealer applicator). The sealer may come with one.

You follow the instructions on the sealer; which will basically tell you to brush a thin even coat of sealer on the the grout with the applicator, let it set for approx. 5 minutes and wipe off excess with a clean towel. You can tell the sealer is ready to be wiped off when the sealer looks like a matte finish as opposed to a shiny, wet finish. Your best bet is to get as little as possible on the tile. It wipes off pretty easy if you don't leave it on too long. Do a section at a time and you will have no problem.

If you have tile in your shower, you will need to seal the grout there as well.

I recommend 2 coats, no matter how good the sealer claims to be. I bought the expensive, supposed to be the best stone and grout sealer and noticed after a few days of showering, that my one coat application is starting to let water soak into the grout.