Ok well i got this from my neighbor, he said "i use to play a long time ago, and i know you play now so i want you to have this," randomly when i was outside.
Only thing wrong is its really dusty and probaly needs a cleaning, and i cant get the pump arm to move.
Theres a screw just in front of the trigger guard, take it out and the stock and brass with seperate. Grab a sheridan valve tool to remove the valve, once everythings out, scrub it down with soap, water and a squeegy.
Take some steel wool to all the rusted metal parts and lubricate everything up with (basically) any liquid lube.
Edit: And some WD40 to try to loosen everything up to you can dissassmble it will be fine, just clean it all off afterwards.