Welcome to the forum!
Find the make/model of the system, probably noted on the reservoir you found. After that, come back here with the info for the forum's experience on your specific system, but there should also be manuals and many forum topics discussing your system online. The correct fluid level is usually within a range shown on the tank, possibly with an external sight tube. My tank also indicates the correct pressure, but time has not been kind to the writing on the tank.
Does your system have multiple steering stations? I recommend starting at either the high or low point in the system and tracing all lines to the other end, looking for leaks or other system problems as you go. The low point is likely at the rudder master cylinder and the high point at the highest helm.
Good luck!
Greg.