The best thing to do is A LOT of research and planning. I you just dive in you could end up with A LOT of issues. If you're not planning on upgrading you want to stick with fish that stay small and aren't so aggressive. So look up shell dwelling cichlids and other dwarfs.
If you are thinking about maybe ugrading you should look into mbunas.They are colorful and there is a lot of variety. They also have great personallities and are fun to watch. If you are thinking that you might upgrade they will sway you into it. I started with one 55 gallon. Now a 55 gallon seems like a fish bowl. I have large tanks in every room on my first floor and the plans for finishing the basement are centered around an L-shaped costom aquarium.