Hi, annielauri,
Welcome to AA! Your new tank is going to have to go through a cycle during which it will grow the good bacteria you need to keep ammonia and nitrite levels (caused by fish urine) low enough for your fish to survive. During the cycle your fish will be vulnerable, because the bacteria aren't there yet, and levels of ammonia and nitrite can quickly rise to levels that will kill your fish.
Some people do a "fishless cycle" to grow these bacteria. You can find links on the site that will describe how to do that. It is also possible to cycle your tank while you have fish in it, but you need to be willing to do some work.
You will need to test your water daily and, as mentioned above, do partial water changes using a dechlorinator whenever your ammonia or nitrite levels go above 0.5. You may be changing part of your water every single day for a while. The entire cycle will take about 6 weeks usually. After it is complete (when ammonia and nitrite levels STAY at zero, and you are measuring only nitrAte), then you can reduce partial water changes to once every week or so (normal maintenance of a fishtank).
Fish can go spazzy if ammonia levels are rising and starting to burn their gills. Otherwise, some fish just really like to play in bubbles.
As was stated above, a male molly will pester a female molly CONSTANTLY for sex. Most people try to buy two or three females for every male, just to make sure the females get some rest.
You need to get a testing kit that tests for ammonia and nitrite at the very least. A nitrAte test kit is also very helpful, so you can see where you are in the cycle. If you keep your fish during the cycle and don't do partial water changes, they will likely die from ammonia and nitrite poisoning.
Do some searching on the site to learn more about the cycle and how it works. You have somedecisions to make: You can return the fish and do the fishless cycle, or you can commit to the work of a cycle with fish. Good luck, congratulations on your new tank, and keep asking questions!