exactly, research fish, find a fish that you must have in your tank (you have alot of species in your tank as it is on the larger size), and then from there, build around that fish figuring out what would be compatible with them and who likes the same tank params as they do.
You have to do some reading to fine some fish that you like best. What I would suggest is find a few center piece fish and then build around them. For center piece fish I would choose one mid-level one and one bottom dweller. For example, I have a 90g as well and have decided that my main bottom dwellers will be a small school of clown loaches (I started iwth 5 but one died so I have 4 now). As my mid-level fish I chose a school of 6 rainbowfish. To that I have some zebra loachs and a red-tail black shark as additional bottom dwellers and some gouramis and zebra danios as additional mid-level fish. I also have some otos and a pleco for algae cleanup.
I've generally gone for smaller groups of mid-sized fish but a large school of smaller fish look quite nice too. For example, a school of 30-40 Cardinal Tetras look very nice in a well planted aquarium. You could supplement them with a school of cories and and maybe a pleco for the bottom and maybe another mid-dweller or two (a school of rummy nose tetras would be a nice contrast to the cardinals and/or some of the dwarf cichlid species) and you have the makings of a very nice south american planted tank.
All I can suggest is do a lot of research both on the web and at the fish store. Look at the fish and the tanks and figure out what kind of fish you like. Pick a couple of favourites and build around them.