A good link for learning about pressure and some practice problems
http://www.chm.davidson.edu/vce/GasLaws/Pressure.html
Part 1: Reading a Manometer
Part 2: Using a Manometer containing Water
Part 3: Effect of Vapor Pressure
Part 1: Reading a Manometer
Part 2: Using a Manometer containing Water
Part 3: Effect of Vapor Pressure