So you've been keeping fish for a while and want to try your hand at cichlids but you're not sure what to get? No worries. You can always Google something like "beginner cichlids" or similar. First, however, it's a good idea to familiarize yourself with this exciting family of fish. There is myriad information about cichlids in books, trade publications, websites, etc. However, If you're a beginner and would welcome some advice, read on.
Tank sizes: Small tanks should only be used for small cichlids. Many species of cichlids get quite large, ~7" plus. Don't purchase juvenile cichlids that will reach 6-7" at maturity if your tank is 10 gallons. That's cruel not to mention irresponsible. My rule of thumb is 15-20 gallons of water per 1" of fish, at maturity. Thus a full grown fish of 7" should have, at minimum, 105-140 gallons of water. That rule doesn't entirely scale, but I think it's a responsible way to plan, especially for a beginner. Nothing is more disheartening than seeing a full grown Oscar languishing in a 20-30 gallon tank, though many irresponsible hobbyists do it. Also consider how many cichlids you intend to house in a single tank. You can often keep multiple cichlids of the same size in a single tank.
In addition, be cognizant of tank dimensions and the species you're keeping. Go with a larger tank footprint, when possible. Footprint = length x width. For example, a 33g long tank has the same footprint as a 55 g tank. Many cichlids will spend the vast majority of their time around the bottom of the tank, so the extra volume of a tank with a greater height might be more aesthetically pleasing than practical.
Water parameters: Not all cichlids (large or small) are suitable for any water. In other words, not all water is equal. Tap water in Boise, ID isn't the same tap water as Atlanta, GA. The liquid environment in which fish live is as varied as land and temperature is for mammals. Wolverines don't naturally thrive in Arizona. Very few cichlids are native to the US. In fact, most are endemic to Africa and South/Central America, and many species will only thrive within specific water parameters. But don't fret about this. There are many beginner cichlids that will do well in most municipal water. Also, many cichlids available in the hobby are domestically bred, where generation breeding may reduce the dependency on native water parameters or at least create a wider tolerance range.
Aggression: Almost all cichlids are aggressive, especially toward conspecifics (their own species). This aggression can be mitigated in several ways, not the least of which is providing them with plenty of space. Most cichlids are territorial, meaning they set-up shop in a pretty defined space that they call their own in which trespassers are treated unkindly. Some of the smaller species don't enforce a large territory, which means more than one can co-exist in a reasonably small tank. You've probably seen some tanks in restaurants and stores that are teaming with cichlids. Housing a large number of these fish in a single tank limits territory claims and distributes aggression, which often prevents a single fish from getting bullied. There are other mitigating solutions, but we'll save those for another day.
Hopefully, this information is useful and will aid your decision making as you enter the fabulous world of cichlids!
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub