Water chemistry is a hard concept at first but really gets easy as you do it.
First decide if you're going to use bromine or chlorine - if chlorine the only stuff to use is dichlor (DO NOT buy Walmart spa chlorine). Both have good points - if you use bromine you need a bromine bank to have a reserve, chlorine can be added before or after using the tub.
You need a test kit or strips, personally I use a kit and once you get used to it - IMO it's the best way to test. The best kit is probably the Taylor K-2005 for chlorine, there is an equivilent Taylor kit for bromine.
I use baking soda to raise PH and alkalinity. They sell acid if either is too high.
Calcium hardness - some people believe in it and some don't. Based on the water testing, if the calcium is over 400 (I believe) you need scale inhibitor. Low calcium water can foam.
PH levels - 7.2 to 7.8, Calcium - 150 to 400, Alkalinity - 80 to 120.
You may want to buy new filters. If not buy a filter cleaner, soak them and rinse, rinse, rinse, rinse rinse, rinse, RINSE. Personally unless you know the person you got the spa from - disinfect the filters - after filter cleaner, soak them in bleach and water solution and rinse. Clean the cover with a bleach and water solution, let dry and use 303 vinyl protector on the cover.
Depending on how clean the tub is ... (assuming the tub is dirty) - fill, shock with bleach/chlorine (high level - 50 PPM), run pumps for about 1 hour, turn every knob - make sure chlorinated water touches everything, drain.
(Start with clean tub) Fill, heat, put in PH stuff when about 80º, let it stay for at least 4 hours before retesting - overnight even better, test PH, add sanitizer, done. You can go in once it reaches close to your set temp without adding anything (think bathtub).
That's it in a nutshell. Oh yeah, I forgot - ENJOY!