first off, people are talking about how high the suspension needs to be where it attaches to the tree. it will almost always be head height or higher. the farther apart the trees, the higher up you will need to attach to them.
the support lines should be running downward toward the hammock at roughly a 25-30 deg angle. you want the hammock at about chair height so you can easily sit down into it.
ideally the ridgeline should not be very tight once you're laying down in the hammock, if it is, it means you need to attach the straps higher on the tree, and loosen them at the same time so the hammock is still at chair height. in other words if the ridgeline is guitar string tight when you're laying in the hammock, it's because you don't have the straps running at a 25-30 deg angle, you need to loosen them a good bit, and since the hammock is at chair height, you'll have to raise them on the tree to keep the hammock off the ground.
most like the foot end whipping to be several inches higher than the head end once they're laying in the hammock
the stuffsac the hammock came in is waterproof
move around to find your sweet spot, but generally i like to be closer to the footbox as it gives me more head/shoulder room, be careful though, you don't want to be so close to the end of the footbox that it compresses the insulation in the footbox of your sb/tq (at least in colder weather)
the guylines on the hammock are your preference, steeper down on the zip side gives a better view, farther out gives more head room.