How do I clean and reseal it.

To clean grout, just use warm water and a brush and agitate the dirt and grime with a brush and wipe away. For heavier stains you can add vinegar to your warm water, and for really deep stains you can add baking soda.

If your grout needs sealing (usually when you notice grout absorbing water as opposed to it just resting on top), pick up some sealer, apply it with a paint brush (wiping away all sealant from your tiles) and let it dry for a couple of hours.