Few questions to start everyone off.
Is your tank cycled? How old is your tank?
What kind of fish are you leaning towards? (Livebearers? Catfish? Tetras? etc.)
What kind of tank are you wanting? (Brackish? Freshwater? Completely Marine?)
What are your water conditions? (How hard is the water? What is the pH? What is the temperature? Do you have a heater?)
What kind of filter do you have?
What kind of substrate/bottom is in your tank?
What are you tank dimensions?
This info will give people a general idea of which fish can best be in your tank for the LONG TERM. Any fish can just be thrown in a tank, you want fish that will be happy and healthy in your specific tank. Everyone's tanks on here can all look the same, but if you pick them apart, you will find that while some may have the similar ideas, themes, or even conditions, there will be something that will set them apart and make their tank "work" with their stock.
Also, if you have ideas, post them here and people will tell you if the fish will work or not.