Okay... without knowing too much about your tank and/or experience, I'll have to assume a few things:
> You've already tested your Ammonia, Nitrites, and Nitrates- which are the parameters that, if at zero (except Nitrates, you will have readable measurements with Nitrate, they just shouldn't be too high) would indicate that your tank is ready for new inhabitants.
> You've already tested your Calcium, Alkalinity, and Magnesium levels- this isn't important for fish, but if you're going to be adding any corals, then this should be checked.
Getting the assumptions out of the way, I'd suggest spending $100 or so and get a small 10 gallon tank from PetCo/Petsmart or something, a sponge filter/airpump, small powerhead, PVC for shelter, and some Cupramine and Prazipro (if you haven't already). This will give you all you need for a quarantine tank and give your new fish a much better chance at surviving. Optimally, all of your fish will go through this process, so as to not introduce any parasites (e.g., "ich") into your tank and infest your other fish.
If you're like a majority of other fishkeepers, you'll ignore that last piece of advice rationalizing that you don't have the money or room, but this is honestly just laziness on most people's parts. So I would seriously consider getting a QT set up and running before even thinking of getting a new fish.
And finally, to answer your question... Regardless of what you do (set up or not set up a QT), here's a list of reef fish that I think are pretty tame and hardy:
> Royal Gramma- common and pretty hardy with a nice splash of color
> Yellow Wrasse (Halichoeres chrysus) or Radiant Wrasse (Halichoeres iridis)- colorful, active and hardy.
> Maybe a Fairy Wrasse of some kind- tons of different colors with these, so you might just want to do a search and see which ones you like.
Once you hone your skills in keeping fish, you could also consider adding a Dwarf Angel down the road (probably as your last fish)... Flame Angels and Coral Beauties are nice choices. But again, these wouldn't be until later on when your tank is more established/stable and you've grown more comfortable in your fishkeeping.
Good luck!
-Chad