Yep. Despite what your LFS says, don't get damselfish. Best answer to that question is to google it, go to an online store and ask them questions via the contact us page. Ask a lot of questions here, but it helps to give people here some specifics, or some choices of fish that you particularly like, and allow the people here to give me the pros and cons etc. There are a lot of awesome looking fisheroos that you can keep in your tank, but it's all going to be personal preference in the end. Look for fish with personality; there is no end to the enjoyment you will have with a fish or two that make you laugh, constantly surprise you and so on. Just watch out for jumpers; the worst feeling is seeing a fish that decided to take a carpet nap. Most fish that you can buy for a reasonable price will be hardy and health, but still require research. Once again, pick out some fisheroos that you like and post a question here about them.
As far as "hardy fish", the best answer to that is "hardy", or healthy water. The best piece of equipment that you own are your own little hands with their wonderfully designed, opposable thumbs! Husbandry of your tank, meaning actively staying in charge of your tank (cleaning everything , keeping your sandbed free of detritus, keeping your tank has algae free as possible by actively eradicating it when you first notice it, etc) is the best weapon/tool you have available to you. When it comes to water treatment, the first part of the solution is always prevention! Stay on top of all that and you don't need to worry about a "hardy" fish because your tank parameters will all be good and healthy enough to sustain life. I hope that helps. Lloyd's $0.02