40 gallon aquariums are excellent for beginners. The reason for this is two-fold. Many of the species and combinations suited for this size are forgiving and can tolerate a learning, yet eager caretaker. Secondly, the size is small enough that maintenance is not overwhelming but large enough to ensure considerable stability.
The following combinations are many and varied; therefore, it does not make sense to comment categorically here on substrate or decorations. Please research each setup's specific requirements. Items to consider are diet, substrate, plants, rocks, and caves. A reliable heater, preferably 150W, is always an important consideration.
There are many methods for filtering a 40 gallon aquarium adequately. Some recommended filters are: AquaClear 500, Filstar XP2, Fluval 204, Emperor 400, Eheim Ecco 2233, or H.O.T. Magnum 250. All of these methods would benefit from an airstone.