Your snail tank should have at least two inches of damp compost or moss for your snails to burrow into. Popular substrates are peat-based compost, coconut coir (coconut fibre) and spaghnum moss. These can be bought at pet shops that cater for reptiles, garden centres or online on sites like eBay or Amazon.
Substrate should be cleaned or changed every month or so to keep the tank fresh. Some substrate can be re-used by baking it to kill any bacteria - peat and coir should be baked at 200*c for one or two hours, left to cool and then dampened before returning it to the tank. Sphagnum moss can be cleaned by rinsing it through with warm, not boiling, water.
As well as substrate, you can make the tank more interesting for your snails by adding pieces of driftwood or cork bark for them to climb on and hide under. You could also try adding artificial plants for more decoration. Avoid hard objects, such as flowerpots or rocks, as snails can fall from the sides or top of the tank and crack their shells on them.