Ideal starter fish are livebearers but they reproduce so not really a good idea.
Next would be fish like Danios and you get lots of different types now. Most tetras are forgiving but not neons. You could try danios, glowlights and some corydoras catfish in a 15 gallon.
A lot of peole keep tropicals in 10 gallons but fish need to exercise and swim so personally speaking 15 should be your minimum, though others will say otherwise.