First – Keep yourself safe. Depending on the size of the fire, it may not be wise to try to get a closer look. Get to a vantage point where you can gather valuable information to relay to the dispatcher.
Then call 911. Location, Location, Location. Provide the dispatcher with as much information as possible (cross streets, mile markers, etc.) to help firefighters find you faster.
If you live outside of a 911 service area call your county sheriff or emergency services department.