Open an access to your attic, and install plywood boards for a walking area. Cut these boards in 2-foot-wide sections, so the electrician can move around a large area. Nail them to your rafters. You might not wish to install flooring in the attic, but your electrician will need support flooring. Hire an electrician who is well-versed on electrical codes in your area. Call local authorities, and get a permit for the work, if needed.
Buy a fixture that you will install at least 6 feet off the attic floor. The floor would be considered the top of the floor joists. If you can get an A-frame ladder into the attic, have the light installed as high as possible on the central peak. Purchase a sturdy outdoor-quality fixture that can survive possible rain leakage. Buy a light with a cover shaped with a wide bottom. This will give the light beam a broad base for shining out in all directions.
Check out the wiring in the attic area to define where to splice the wiring for the light fixture. Only a licensed electrician can assess an acceptable juncture for splicing. In some cases where you can find no acceptable splicing point, you will need to run the wiring all the way from the breaker box and up the walls into the attic.
Secure the light to a high beam in the attic with appropriate metal fittings. Install a bright bulb. Install a light switch next to the attic access door or hatch, and paint the cover with fluorescent paint. This will make the switch easier to find in a hurry. If there is a nearby side wall in the room below the attic door, run the wiring for the wall switch to this wall. Turn the light on before you climb into the attic space.