I would start at the standard PCR reaction mix that is displayed by your PCR kits manufacturers instructions.
Go from there, and then you can optimize the annealing temperature of the primers.
So the most important to least:
1) Try a standard PCR with standard annealing conditions (can estimate your primer annealing conditions using various primer programs)
(usually you get a nice band right away with the standard conditions)
if not try this:
2) Optimize annealing temperatures. This is usually the most important optimization. You can do this with a gradient PCR machine.
3) Optimize difficult PCRs with PCR adjuvants such as DMSO.
DMSO can work for difficult PCRs and GC rich PCR. You can try 1%, 2%, and 4% DMSO after you optimize for annealing temperature.