I'm doing A-level biology also so i'm in the same boat as you, I'll try to make my explanation as clear for you as possible. Firstly you'll need to know about osmosis in order to get to grips with water potential (Ψ), osmosis is a type of diffusion (in water) where the net movement of water moves from a high to a low water potential across a selectively or fully permeable membrane. In simple terms, this means that the number of water molecules try to evenly distribute each other at either side of the membrane. In order to do this, the side with the high amount of water molecules (higher water potential) will need to move to the area of lower water potential to reach equilibrium. Ψ is measured in kPA (killer pascals), 0 is the purest form of water (such as in deionized water) with pretty much no other solutes in solution. Lower water potentials (-100kPA,-200kPA) have an increasingly greater amount of solutes in solution. Sometimes I get confused with the units so I try to remember it this way, you can't have positive values with water potential so its always either negative or zero, so think the more negative a number the lower the water potential is.
I'll give a practical example, if you submerge some equally sized samples of potato into a concentrated sample of sucrose solution (A) and the rest into a sample of deionised water (B) for ten minuites or so you would see a change in mass. The potato in sample A would lose weight and the potato in sample B will gain weight, your probably wondering why this has happened so try to remember the definition for osmosis (diffusion of water) where a high water potential will always move to a lower water potential. In sample A the potato was submerged in concentrated sucrose solution that has a low water potential (not many water molecules, mostly sucrose) This water potential in the solution would be lower than that in the cell (which is also slightly negative because of the sucrose used as food in the potato) so the net movement of water will move out of the potato through its membrane into the sucrose solution and so the potato will lose weight. The opposite happens to the potatos in sample B, where the water potential in the deionised water is much higher than in the potato (almost 0) so the net movement of water will move into the potato in the hope of reaching equillibrium (which it never really does) and so the weight of the potato increases.
I hope these definitions and the practical example has helped clear up the confusion with water potential :)
Solute potential is basically the same as water potential, but it refers to the amount of solute in solution rather than the amount of water. So the net movement of solutes will always move from a low to a high solute concentration across a selectively permable membrane. Pressure potential describes the density at which molecules take up a particular amount of space, in high pressure their are alot of molecules in the same amount of space and in low pressure there are fewer molecules roaming in th same amount of space. So the net movement of these molecules will always be from a higher to a lower pressure potential.