Can an external light fixture and an external receptacle share a circuit?
Should you? No, Greebo is right (+1), lights and outlets should be separate.

Can you? Maybe, it depends on whether the power source is at the light fixture, or at the light switch. If it's at the light switch, then your outlet would also be switched.

What do you need to know? Everything about electrical safety and codes so that you don't electrocute yourself or create a dangerous situation. Things like turning the power off at the breaker, testing the lines before you touch anything, using a GFCI outlet, using an outlet cover for exterior protection, stapling the wire to the stud, using the proper gauge wire, etc. If any of that list leaves you scratching your head, it would be best to hire a professional.