I got a 5 gallon because I think it's a good beginners tank
Mistake number one.
So my fish tank just arrived,and it's now time to fill the tanks up with fish.
Mistake number two.
The smaller a tank the harder to take care of it. In a 5g the smallest water quality issue will be enough to wreck the whole tank, be it an algae bloom or a fish die off.
The larger the tank the more leeway you have with problems. 10g would be the absolute smallest I would ever recommend for a beginner, 20 would be even better.
Next - if you're going to "fill up the tank" now all your fish will be dead within a week (it's called New Tank Syndrome). The nitrogen cycle needs a few weeks to establish itself or you will have of the charts ammonia levels.
What you have to do is let the tank run with water only for a week, then add one or two hardy fish. Keep them there for at least 3 weeks. Only after that could you start to stock the tank with the fish you want - one at a time.
Mind you there's not much you could keep in a 5g, maybe a guppy and a couple neon tetras.