Welcome!
If you don't make any blunders with the chemistry when you get things set up initially and maintain things, your pool can stay sparkling clear all season long very inexpensively. After buying the test kit and the pole and brushes and so on -- which are a one-time thing -- you should easily be able to maintain it for under a buck a day over the course of the Summer.
18' * 48" is no kiddie pool. You need a grownup test kit for that grownup pool. It's a chunk of change, but it'll last you all season and probably a good way into next year, too. And if it saves you one algae bloom, it'll pay for itself.
If you haven't already ordered a kit, do it now. You won't find what you need in stock at a pool store, almost guaranteed. It'll have to be online most likely. You can read the Test Kits Compared in pool school or skip it and head over to
TFTestkits.net and get a TF100. It's only coming from the next state over, so even with the holiday you'll have it before next weekend.
I urge you to spend a little time in pool school. It should answer questions you haven't even thought to ask yet. Start with the
ABCs of Pool Water Chemistry
A tip: if the pool came with any kind of startup kit that includes dry chlorine of any sort, tell us what the label says and we can advise you on how much you can safely use before you overdose on the CYA byproducts.