UPDATE: As of the release of the Ice and Steam DLCs, this cheese method now seems to have been, if not completely patched out, made significantly harder to use.

You can jump up or across to higher or farther ledges and, as long as your hands or arms are over or just on the edge of the ledge, you can move the right stick down to pull yourself up on top of the ledge. This works the same for ledges higher up. If you hold your arms above your head and place them on top of a ledge, move the right stick down and you'll pull yourself on top of the ledge if your arms are wrapped over or just at the top of the ledge. Now climbing isn't introduced until level 4 - Mountain, but it's definitely worth knowing about. For example, if you need to press a button to open a door, hold your arm out and look at the button to get your arm level with the button. Once they're raised, you use the right stick to look around, and your arm will position itself where it's looking.

You use the left trigger to raise your left arm and the right trigger to raise your right arm. If you pick up one of the first few walkie talkies, controlling the positioning of your arms is introduced and it's a very simple topic. Here I'm going to outline the key tutorials the game will teach you, just so you don't have to waste time in game looking at them and get familiar with them here:

How to climb in human fall flat