How do you use thevenin's theorem to calculate current in resistors?

If this is an independent current source, it has to be disconnected.

Independent voltage sources are replaced by a short-circuit.

More about this at (see Related links):

MasteringElectronicsDesign.com: How to Apply Thevenin's Theorem - Part 1, Solving Circuits with Independent Sources

and

MasteringElectronicsDesign.com: How to Apply Thevenin's Theorem - Part 2. Nested Thevenin Sources Method