Most garden gates are installed between vertical posts so the gate swings inwards. As the hinges can be installed to either post, the gate can swing in from either the left side, or the right. The gate fastens to the post opposite the hinges with a metal latch at the top, or a sliding bolt. Though latches keep the gate in the closed position, the sliding bolt's barrel has to be slid horizontally to lock the gate, providing less chance that the gate will be opened accidentally.
Stand inside the gate and familiarize yourself with the latch or sliding bolt. Open and close it many times, so you know the location and movement of its handle or lever.
Walk outside the gate, pulling the gate closed behind you--if a latch has been installed, the latch will lock into the closed position automatically. However, if the gate has a bolt, reach over and push the sliding bolt's barrel into the closed position.
Stand near the gate post that houses the latch or dead bolt. Reach over with the hand that feels most comfortable--this usually depends on whether you are right or left-handed. Feel for the latch handle (if you can see it from where you are, all the better). Lift it up into the "open" position, and push the gate open (away from you) with the other hand. If you have a dead bolt, push the barrel handle horizontally away from the post into the "open" position, and push the gate open with the other hand.
Practice opening the gate from the outside, always using the hand that's easiest for you to reach the latch/dead bolt. It will become an easier process the more you practice. For latches, it is possible to attack a small chain or string to the latch handle, and feed it through (or over) the gate. The string/chain can then be pulled from the outside to open the gate. Though this is not as aesthetically pleasing, it is helpful for people with flexibility problems who find it hard to reach over the gate to open it.