Hi NathanJames and welcome to TFP!
Test kits hold the saying: 'go cheap', and it will cost you way more in the long run. There are countless postings here from people struggling trying to fix their pool with the cheaper test kits.
The TF-100 kit is the Snap-On tool of fixing, maintaining and preventing pool water problems.
I would suggest the XL option because of you already having a bad ugly green pool!
All the info and help you need is here.
Stay out of pool stores and get your own test kit and follow instruction here:
just-getting-started-f24.html
Go here for kits:
If budget is a concern, get the TF-50 here:
http://tftestkits.net/splash-page.html
But with a green pool you may have to reorder supplies all depending how bad your pool water is and how efficient you learn to do this methods taught here.
I did have the same problem when I landed here 1 1/2 years ago.
I was very apparent to me I needed all the help I could get, this was all very overwhelming at the beginning. Now it has become so elementary and I'm now helping newbies like yourself.
By the way you just got some of the best advice ever... from all those here that taught me!
My pool does exactly what it's told to do now and I always have a perfect pool all the time screaming every minute to come swimming!
Chuck