Well, you'll need to cycle it first
this takes 4-6 weeks, which will give you lots of research time
In a 20, you could have danios - they come as long fin, short fin, leopard and zebra - generally cheap, but will need a heater. They do best in groups of 4-6 or more.
There are also neon tetras, skirted tetras, barbs of all sorts...
My question is... what do you like? fast or slow fish? colors or some...interesting fish (blind cave tetra :o )...?