Home Depot sells two basic types of grout sealer in
plastic bottles. One is a yellowish color and the other
is a dark brown (I think). The darker bottle is about
double the cost of the other. It's about $15 and claims
about a 20 year wear rating.
Get this better quality one.
Also get an applicator bottle. It has a pointy top with
bristles that look like a small paint brush.
You fill this bottle and holding it upside down, you drag
the brushes backwards, releasing enough sealer to
fill the joint as you go. Don't get it on the tile...if you
do just wipe it off.
It will absorb in in about 10 minutes and be "dry" in about
1/2 hour. It takes 24 hours to fully dry before you can wash
If you've used a polymer based grout, one coat will do it.