Sorry, but this might get lengthy, so bear with me please :)
[pH](http://socratic.org/chemistry/acids-and-bases/the-ph-concept), or potential of Hydrogen, is measured on a scale from 0 to 14. 0 being the most ACIDIC, while 14 being the most BASIC.
To find pH from the concentration of H_3O^+ (or just simply H^+) you need to do the following:
pH= -log[H_3O^+]
The [H_3O^+] is just the concentration (in molarity) found through calculations (I'll cover that soon)
If you have the concentration of OH^-, however, simply find the pOH by:
pOH=-log[OH^-]
After you get this number, you do the following:
pH= 14-pOH
OK, so lets start with the basics of determining your H_3O^+ or OH^- concentrations. Molarity is the standard unit for concentration in chemistry, and is simply moles of substance over liters of solution.
M=(mol)/(volume (L))
So whenever I say concentration, I mean Molarity.
You find the concentration of H_3O^+ by first writing out your acid dissociation equation:
HA+ H_2O= H_3O^+ + A^-
...where HA is simply the acid you're dissolving in water.
If you have a STRONG Acid, then it dissociates (dissolves) completely in water. The concentration of H_3O^+ is the same as the concentration of the initial acid.
Now, you were probably given the Ka of the acid, telling you that it is a WEAK acid. That means that it does NOT dissociate (dissolve) completely in water. The Ka at this point is just a number to plug into your equation.
To find the concentration of H_3O^+ from the Ka and your equation, simply plug the numbers that you have into this:
oi59.tinypic.com
You can do the exact same thing if its a BASIC solution, just replace [H_3O^+] with [OH^-] and dont forget to change the pOH to pH