Specific answer:
1. Check your water parameters with API Master Kit for fresh water. 0 ammonia, 0 nitrites, <40ppm nitrates before you add any more fish.
2. Make sure any disease is gone from your system before getting any new fish, aka wait two weeks.
3. Get two nerite snails and five white cloud minnows, or five tetra, or five Endlers, or five danios of your choice. Take the time to float the bags in the tank ten minutes. Open bag. Scoop out 50% of water and discard that water. Do not discard it in your tank. Scoop 1/4 cup of water from your tank and put it in the bag every five to ten minutes for half an hour to 45 minutes. Scoop fish from bag water and add to your tank. Throw away acclimation water and bag.
4. Maintain your tank by vacuuming a different 1/4 to 1/3 section of tank floor every week. Add dechlorinated or reverse osmosis/distilled water to replace what was lost during cleaning. Use API Master test kit to make sure your ammonia, nitrites and nitrates stay 0, 0, <40ppm. If they creep up, use the siphon to remove 25% of water, replace with aforementioned types of water until aforementioned levels are achieved.
5. Enjoy your aquarium. This last part is critical, else there's not much point and no motivation for 1-4.