Most Ortho or Time Life or Sunset "How to build decks" books that you find in Home Depot or other home improvement stores will include span charts in the back. The span charts illustrate the recommended maximum span for any specific type of lumber under specific usages.
As a rule, unlike Steve, I prefer to space my joists every 12" oc, because it doesn't usually add more than about $10-$20 to my total material costs, and it does wonders towards stiffening the deck structure.
The entire frame of the deck should use Pressure Treated lumber, the posts should set ON concrete post footings that should extend at least 3' into the ground with a SIMPSON STRONGTIE post bracket set into the concrete.
I also prefer the deck joists to be AT LEAST a 2x8, if not bigger. But then again, I build decks in SEATTLE, where wood rot is a daily experience, and I prefer my work to outlive me.
If you have specific questions about this, feel free to post again, or email me directly.