Which tropical fish shall i get?!?

Neons require a school of six or more, but your tank seems a bit too small for schooling fish. You could get 2 or 3 guppies, plus a couple oto algae eaters and maybe a java fern to add oxygen and help regulate algae growth. That size tank would also be good for a single male betta (don't put guppies with male bettas--the long, flowing tails could induce aggression from the betta). Neon tetras can live in 68 to 74 degree water, while guppies have a wider temperature range of 64-82. It would probably be better to have your fish in the warmer side of this, so a heater is recommended.