There are several water testing kits on the market. Make sure they include tests for: Ammonia, Nitrite, Nitrate, General Hardness, and pH. When you are cycling your aquarium you should test every day. Once it has been cycled you can test it once a week.
The only common digital tester I know of is for pH (not counting things like TDS, ORP, conductivity and salinity).
The paper test strips, in my experience, are less precise than liquid tests. Many people think they're less accurate, too. In addition, paper test strips are more expensive, per test.