A few suggestions that are, at this point, moot: Research fish before you buy them. Fish stores will give incorrect/bad advice on fish, and you could end up with a bad tankmate, a fight that grows huge, or something that has very strict dietary needs.
Quarantining fish before introducing them to your livestock is really important, as is slowly adjusting the fish not only to the water temperature but the water parameters in your tank.
On to what you can do now. First of all, don't worry too much. Most fish need time to adjust to a new tank, and some particularly skittish fish (like many cats) won't eat for the first day or so. I had kuhlis that wouldn't eat (at least while I was around) for three days. It's likely your new little friend is just timid because of new surroundings.
These little guys generally eat insects from the surface of the water, so when it gets accustomed to the tank, things like bloodworms, daphnia, and other such creatures (live, frozen, or freeze-dried) can be added to the diet (it will likely learn to eat flakes) to keep it healthy.
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