default functionality aircraft (includes downloaded ones) are generally easy to start up from cold... basically the checklist is as follows:
Battery - ON
Avionics Power - ON
at this point everything in the cockpit should be lit and working... the engine startup procedure varies depending on the sort of aircraft...
for Props:
Mixture Lever - 100% rich
Magnetos - Start
for Jets:
Fuel Cutoff Switch - RUN
Engine Starters - START (one by one)
DC3:
Mixture - 100% Rich
Engine Selector - LEFT
Engine Starter - Start
Engine Selector - RIGHT
Engine Starter - Start
that's all i can remember now... but it should get pretty much any plane up and running...
another trick you can try, is, if you're having difficulty starting an aircraft, you can switch to a simpler plane such as the C182, and start 'er up... then switch back to the other aircraft and it'll load with it's engines on

this also works if you can't find a switch in an aircraft
Oh, just remembered... if all else fails... hit CTRL+E ;)
Cheers