Heya and welcome!
Before you add anymore fish, you need to get yourself a water test kit. These kits test for common chemicals found in tanks - some of these chemicals will kill your fish if they get out of hand!
The best test kit to get is probably the API Freshwater Master Test Kit. Shop around a bit. I got mine for about £15. It seems expensive but as it makes the difference between life and death for your fish, it's essential!
Once you have that, you can start testing your water. Once you add fish, test the water every day until it is stable again. You will see three chemicals rise - ammonia, nitrite and nitrate. Ammonia and nitrite must have gone back to zero for at least a week before you add new fish. They
will rise when you add new fish and you need to do as many water changes (between 10% and 90%) as necessary to get the levels to zero. Keep testing until the levels stay at zero without your help and you see the nitrate levels rise.
This process usually takes at least two weeks - hang in there! It's important to be patient.
Read these resources - they'll really, really help you along.
Cycling your tank
Beginners' Resource Centre
In terms of fish you can add, once the levels of ammonia and nitrite have gone back to zero you should think about adding some hardy fish such as danios, more platys, harlequin rasboras or a pair of small gouramis (dwarves or honey are probably best). Obaviously don't add them all at once! You'd want to add about three or four more small fish, or only two if you get larger ones.
In your tank you don't want to have any aggressive fish (headache for new fish keepers!!) or large fish (3 inches max size is best). There are some bigger fish you could have but these are the exception rather than the rule. Always read up on new fish and get some advice before buying - you don't want to see something pretty in the shop and then realise you have a 2 foot long monster on your hands!