None of the sand needs to be live sand, but you'll jumpstart your bacteria bed if you seed your sand with some from an established tank.
The number of fish is dependent upon several factors including the adult size as well as the swimming habits and territorial nature of that particular species. Figure out what kind of fish you'd like, and we can help you through the ins and outs.
When you add top off water (to make up for water lost through evaporation) you add freshwater (no salt). When you do water changes to remove pollutants that have accumulated in the aquarium, the water you add back in will need to be saltwater. I'm assuming you're planning on saltwater since you posted in this part of the forum.
Don't forget to buy yourself a good test kit. Most folks around here recommend the API saltwater master test kit for it's relatively low price and reliability.
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