I used to use the mardel test strips when i first started but I had so many issues at first. I was testing all the time and burning through the tests really fast, for a little more I bought the aquatic pharmacuticles master test kit, which uses test tubes and provides al the tests I'll need for a long time, I picked one up at petco for 25 dollars.
As far as how often, if things dont look good, i'll test every day while i do water changes, other than that usually once or twice a week, but I'm fairly new so I like to keep a sharp eye on the water.
Generally I only test Nitrite, Nitrate and ammonia. If the PH is off, there isn't alot I could do and i fear that by changing it back to where it was I could do some harm to the fish. but others might disagree.