Browse around on the internet, look at stuff that people are always asking for, look for stuff that has a high price tag. Figure out what is desirable and rare, by this, it's usually some kind of uncommon strain of an already popular fish.
Also, just because something is rare doesn't mean that it's desirable.
And finally, price is important, to get started with a rare strain it's going to take a decent investment and then some luck to get a good breeding method down. Zebra plecos are a good example, they are awesome and everyone wants one, but only a few people are willing to pay $150-200 for one. But then there are leopard frog peckoltia plecos which people also love, and more people are willing to pay the $15-25 price tag.