Guitar positions
As we come in an infinite variety of shapes and sizes there is no particular right or wrong way to hold the guitar other than what feels the most comfortable.

When standing, make sure that the strap is securely fitted with the strap buttons on the guitar pushed right through the slots on the strap ends. These may be quite stiff at first but will loosen with wear. Some players prefer a high strap position, others prefer it lower. The strap is adjustable so you should experiment to find the most comfortable. You should aim to be able to reach all parts of the neck without any undue contorting of your wrist (on the fingering hand) and not feel cramped in the forearm or shoulder of the picking arm.

Similarly when sitting, you will find that the contours of the guitar fit nicely over one leg and leave your hands in a comfortable position.
A point to note here is that when seated, your wrist will automatically be in a different position around the guitar neck to when standing so if you expect to be playing mostly standing then it is worth practicing that way.