How to reduce Computational Diffie–Hellman problem and Decisional Diffie–Hellman problem to Discrete logarithm problem I'm supposed make some reductions but don't even know where to start. Any help would or explanation on how to do this would be much appreciated.
A reduction here would mean showing that if you had an efficient algorithm A to solve the discrete logarithm problem, then you could use that to construct an efficient algorithm B solving the computational Diffie-Hellman problem. (For starters, assume A always works and construct B that always works. For full credit, given A that works with some probability [over a uniform input], construct a B that works with the same probability [over uniform inputs].) 

If you understand what the discrete logarithm and computational Diffie-Hellman problems are, a reduction should be immediate.