First, you should wait before you add more fish.
It's hard, I know, but trust me.

Your nitrogen cycle has not been established yet,and if you add too many fish you tax the system too much and that leads to disease and death.

The next thing I would purchase would be a test kit for measuring pH, ammonia, and nitrites.
You should be testing the "new" water daily.

This will keep you busy at least, and will help you further understand water quality.

When I first had guppies, we only went to this shop that was owned by an older gentleman.  He had a rule for us, he would only sell us THREE fish at a time. (Even if we begged)

One friend, one day had me get three for him, and he got three too, well the ending is a sad one, so I won't continue.

This will also give you some time to research what you really want to add to your tank.

You would need something to clean the bottom of leftover food.  Hundreds of possibilities there.

But most of all, remember, a male + a female guppy = lots of babies!!!!!

Take care,