If I remember correctly my first mossberg had a wooden dowel in the magazine tube. This limits the rounds in the magazine making it legal for certain hunting purposes. Remove the barrel and just shake the gun with the tube facing down. It should fall out that little threaded hole in the end of the magazine tube.
Some Mossberg have a magazine spring retainer. A metal plug at the muzzle end of the mag tube that must be pulled out in order to remove the magazine spring and then anything other than the follower and spring can be tossed.
YouTube might be beneficial. Perhaps meet up with a member here and learn how to operate your shotgun in a safe environment, then head to a range to make sure all lessons were learned.
And since it sounds like you are pretty new to these (we ALL were at one point), do NOT leave a round in the chamber, even around the house. If is dropped, there is an above average chance the firing pin can ride forward just hard enough to make the primer activate and have a 'boom' when it should only be a 'thud'.