research
In this site are Species Profiles, that is a good place to start.
Big cichlids like Oscars have specific sorts of fish they can mix with. Delicate cichlids like discus are best with only other discus or perhaps small tetras of certain sorts that can handle high temps. Discus are not for beginners, IMO, way too expensive and may tend to bet ill too easy.
What sort of tap water do you have? That makes all the difference, for some fish need hard water and some fish need soft water and many can adapt but have strong preferences. Do you know your pH, KH and GH? Keeping the fish that are best in your water is a good place to begin!