I have a single handle shower valve by Moen. Water is slowly leaking past the valve when shut off. I got the handle off but need to know how to further disassemble so I can replace whatever seal or o-ring needs to be replaced.

A bit more information is needed; is this a Moentrol or a Posi-temp valve? You will need to shut water down to the shower. If it is a Moentrol, it could be an issue with either the cartridge or the pressure balancing spool. If you are at least getting water, I am going to assume that it is a Moentrol.
You will need to turn water off to the valve. Pull the cartridge and examine as to whether or not debris is blocking the supply holes of the cartridge. If your valve has built in shut off screws, you can slowly turn water on to clear any debris that may be inside the inlet port. I typically use a piece of PVC to go over the valve body so that water does not leak between the wall and the shower. Again, just turn water on enough to start it flowing.
If you do not notice anything amiss with the cartridge, it could also be the pressure balancing spool. If that is on your valve, it will be a flat head screw about the size of a nickel. Water will need to be off for the removal of the spool as well. Again, you will be looking for debris that is blocking water flow.
If this is an older Moen faucet, you may just want to replace the cartridge while you have it removed. If it is a push/pull, it is a Moentrol. If you have to turn the handle for water to come out, it is a Posi-temp. In my area, the cartridges run from $20-$40. However, Moen does offer lifetime on their cartridges. Lately, I have had a few more hoops to jump through to get the replacement cartridges from Moen, but they are still very pleasant to deal with.

Moen and all shower valve manufacturers follow american federal guidelines. Hold the unit in your hand with the handle facing you - the left side is the hot water inlet and the right side is the cold water inlet.

Call Moen directly. They will ship what you need to your door, for free.

During construction did you perhaps close off a shut off valve within the water line and not reopen it? If not do you hear anything at all when you turn the shower faucet on? Let me know.

Call moen and give them your model no. they will send you a new cartridge free of charge. if you can't wait local stores like lowes will have them. Turn off your main water shut off valve and take off handle, then with some pliers take off brass ring, the cartridge will pull out. First notice the notch it rests in and notice where the hot water adjustment disc is. Assemble the new one the same way, put some vasolene on orings and reassemble. hope this helps.

Replace cartridge and insure that the C clip is not missing.

Try replacing the cartridge first.That's your best bit . Next would be the whole valve. Hopefully, it's just the cartridge.

Single handle shower controls always have some water trapped inside. It has probably freeze damaged the cartridge

make sure when you turn on the hot and cold shut off valves all the way and turn the moen on after that. Some times I would get that problem if I turned one(either hot or cold) off then tried the shower.

take out the cartridge from the fixture and make sure it has all of the "O" rings and stoppers.
look at the instruction again, make sue you are installing it the right side up. if all that, the shower still leak, then you need to talk to the place you brought the fixture, it may have a defective cartridge.