Ok, here are some tips:

Remember that the fish are animals, not just ornaments. Look after their needs first, looks second.
Buy a reasonable sized book on fish keeping and read it all first, before you buy anything.
Not all fish live in the same kinds of water. Find fish that suit your water best, or they will die.
Give the fish plenty of room.
Don't put the aquarium on furniture not designed for it. When the tank is filled it will be very heavy, and will damage/break the furniture.