Don't be freaked out. Everyone kills from time to time. When I look back on what I've done, and what I know other people did to fish before they knew better, it's really horrifying. My first betta, who lived in a JAR, used to get his whole tank run under the faucet once a week WITH HIM IN IT. He still managed to make it three years like that, the poor little dude.
Check out AQADVISOR for stocking levels and guidelines. That is, how many fish in total you could have. It also tells you if you have selected fish that don't get along or have special needs (like a lid to keep them from jumping out.)
I've been told to add one or two fish at a time, and give them a few days to adjust, as well as for the bacteria to catch up. If you seeded your tank with established bacteria the cycle takes two weeks or so, from what I understand. If you didn't (and you'd know if you did) a month is safe, I think.
Get a test kit if you can. The liquid test kits are more reliable, but even strips are better than nothing. Strips go bad if they're unsealed, but I get the 5-in-1 kits that have a separate ammonia strip, and the strips are individually wrapped.
Feed your bacteria. Add some fish food once in a while to create waste that will become ammonia to feed your cycling bacteria. Add plenty of things for the bacteria to grow on as well. Your substrate (rocks and sand), plants, deco, logs, and plenty of filter media. The bacteria don't live in the water, for the most part.
And maybe add your fish when you will be able to observe them for a good amount of time. You might even think of quarantine (even in a bucket) for a few days when you buy your fish to make sure you haven't gotten one that is incubating a disease. Wait a few days or a week in between additions to make sure everyone is getting along, and no one is sick (if a fish is getting sick at the store, the stress of the move can cause them to start showing symptoms).
I think guppies are a great choice to start out with, they are even easy to breed, and you can find all kinds of different varieties.
Also, if you are buying fish from a chain store like PetCo or PetSmart, don't go by the information on the price tag. This is a good guideline, but inaccurate (for example, Mollies do best in brackish water, I found out the hard way). I would suggest you go window shopping, write down the fish breeds you like, and research them.
I'm just beginning myself, but this is what I've learned so far.