You did not state whether you have a load panel in the garage or not, but either way it is about the same.
For a single 15amp duplex outlet you could run AWG 14 wire but for the few pennies difference in wire cost I would prefer to use AWG 12.
12/2 WG type UF romex (type UF=underground feeder) is suitable for direct burial.
I would begin by digging a trench from the point where the wire will exit the garage to the point of demand at the patio, digging the trench at least 6" deeper than your average frost level.
On the patio end I would set a post from the bottom of the trench up to the height were you desire the outlet.
Install a PVC weatherproof outlet box with covers on the post and run conduit down the post until it is at least 12" below grade.
If your soil is rocky i would suggest backfilling a few inches of sand on the wire to pad it before backfilling the soil.
On the garage end I would then run another lenght of conduit from the trench up the garage wall to the point where you are passing the wire through the wall.
Install a pull type elbow on the top of the conduit and a short length of conduit as a sleeve through the wall.
After you begin backfilling you should run a strip of yellow plastic marker tape about a foot underground and parallel to the wire to warn people in future that their is a buried wire there. (You can get the marker tape at all hardwarestores or home supply stores.)
If you have a load panel in the garage that has provision for another breaker you could install a 15A GFCI breaker to feed the run.
If you do not have a load panel you should be able to tie it into the general lighting outlet circuit in the garage but make sure the garage circuit is GFCI protected.
Linear Mode
