I will offer up a Welcome aboard first! Now take yourself down the forum listings to the Reference section and do some nosing in there, you'll find posts on dating the rifle, in gunsmithing how to take your rifle apart, and stickies at the top that help point the way to various problems and remedies.
D-- makes your rifle a 1947 YOM found here:
I believe that this will help with maintenance even though your rifle is older things haven't changed much:
and as to the lever not staying in place, either the lever plunger is depressed and not sticking out of the lever or the catch bar it's suppose to engage is missing and /or bent. pic here: https://www.marlinowners.com/forums/i...c,32010.0.html
a pic or two would be appreciated and with that I'm sure others will chime in shortly. Mr fixit