No intention of sounding like a dick, lol, but have you tried Google?
There is a wealth of information on many forums and ESPECIALLY on youtube on how to do a proper disassembly. The Marui (yours is a CYMA clone of this) version M14 is probably one of, if not THE, most tricky gun to disassemble - at first until you're familiar with it. Take your time and remember the order of the screws, as some from the gearbox are a different length than others. Also don't over-tighten the screw on the each side of hand-guard that screw onto the small metal piece on the barrel or you will have a bad day.
Along with those videos, definitely watch all the other common ones like how to shim, correcting your AoE, compression, and others.
When it comes to barrels, the Marui M14 ones are cut differently. Prometheus makes barrels for them, but many other companies do as well like ZCI. You are also ok with a 400mm, 407mm, or whatever - as long as it's cut for an M14, or has multiple cuts for any gun to use
like this one (notice how it has two slots on the left - one for marui m14 type hopups and one for regular aegs). If it's longer than 420mm you'll obviously have to worry about hiding the extra length with a suppressor or something.
You may want to look into getting a nozzle with an internal O-ring as well.
For accuracy, it takes time... knowing your gun, testing, etc. A good start for your M14 would be a Modify Baton Ryusoku hopup bucking set (it's a flat hop style), paired with a Prometheus or similar stainless steel barrel. For M14s, you ill have to cut the bucking to fit. Super easy though - just measure it beside the original one the gun came with and cut. FPS consistency plays a big part too - since you want your rounds to end up in the same place each time.