Sitting is not too complex but is used for many things in Second Life. Quite apart from simply sitting on the ground, a prim or in a chair, this is how you will use a vehicle, any pose ball such as a dance or to board a vehicle and even for some short range teleport objects.

To sit on something, simply right click on it and on the pie menu select the Sit Here menu item. To stand up, click the Stand Up button near the bottom of your screen. Sometimes the "Sit Here" text will have been replaced with something else, like "Dance", "Ride" or "Teleport".

What happens when you sit depends on the kind of thing you clicked on:

1) If it has been scripted, it will sit you in a specific place, with a specific pose,
2) If it has no script, you will sit where you clicked, facing in the opposite direction to the direction you were facing when you clicked
3) If you clicked on the ground, you will sit on the ground, facing the same direction you were facing when you clicked

Often you will see a pose ball on a scripted piece of furniture. In this case use this as the target for your first right click and it will automatically put you in the right position when you sit.