Check out Crowley marine. They have GREAT exploded diagrams for Yamaha motors.
Here is the link for the motor you mention. Check out the "POWER TRIM TILT ASSY 1" diagram. Part number 23 is called the "Reservoir Plug". Its really not a plug as much as a cap that screws onto the reservoir. This the where you will need to check and fill the fluid. Before you remove any plugs, caps, anything...make sure you tilt the motor up high enough to engage the tilt locks. This takes pressure off the hydraulic systems so you can open the system w/o getting a shower. For reference, I have attached a picture of the tilt locks below.
Flush is a much more involved process, so I'd start with fill and check for leaks. Make sure the boat is level. Tilt the motor all the way up until it rests on the tilt locks. Remove the reservoir plug. Fill the reservoir until the Tilt/Trim fluid begins to leak out. I use Yamalube Performance Power Trim & Tilt Fluid. Make sure to clean and lube the pistons (there are three of them. two small one large). I use standard 2-4-C grease. If your motor is very noisy when being tilted, make sure to lube the friction points. There should be 4 grease fittings, two on each side of the motor where it mounts to the jackplate.
Finally after you've run the motor a time or two, check for leaks. The most common place to find a leak is at the base of the two smaller tilt pistons. This is generally due to dirt and grime on the tilt pistons being sucked down past the seals. If this happens, its a pretty straightforward issue to replace the seals and o-rings. Check back here if needed and I'm sure someone can step you through the process.
HTH