There are many ways to charge a keg. Here's a few:
1) You can "force carbonate" with CO2 (10-15 PSI for 6-7 days on a cold keg) at which time the beer will be ready to serve. Note: There are also a handful of ways to speed up this process, but the impact on the beer (head retention, etc) is hotly debated.
2) Dissolve 2-3 oz of priming sugar in 8 oz boiling water and add to full keg. Let keg sit at the same temp as the last stage of fermentation for 3-4 days, then chill for a few days so the CO2 fully/evenly absorbs. You don't need as much bottling sugar for this method as the usual 5oz (or so) when bottle conditioning a 5 gallon batch.
3) Old-Timey krausening method which works great: Scoop 16oz of wort from the kettle at flamout and freeze it in a sanitized container. Let it thaw overnight before kegging day and add this "spiese" directly to the full keg. This takes the place of the bottling priming sugar in #2, so follow the same temp/time directions
#2 and #3 should charge your keg nicely. You will probably still need to apply CO2 to push the beer out, especially as the keg becomes defective (drained).