Welcome to AA!
If you don't own one already, you should pick up a test kit for your tank (get a liquid test kit, not one that uses strips). Test your water parameters every few days. Ammonia and nitrites are very toxic to your fish; nitrates are less toxic but still should not build up too much.
I would make sure you're replacing 20 - 40% of the tank's water every couple days. This will minimize the amount of waste that your fish will be having to swim in. Whenever you do a partial water change (a
PWC), siphon the old water out and replace it with dechlorinated water (you can buy product that will remove chlorine (also toxic to your fish) from the water).
Since your tank has too many fish in it, they will be producing a lot of waste that they'll be swimming around in and will have weaker immune systems as a result from the stress. To keep them healthy, make sure they're in clean water! Once you get your bigger tank, caring for them will be quite a bit easier.
Be sure to ask questions as the folks here at AA are all very knowledgeable people on this hobby. Good luck to you and your fish!