You did not tell us the make. Contact the maker (most have a website) and ask for an owner's manual (they are free). You can get a decent .22 cleaning kit at any sporting goods store.
YOu must be 18 and have a clean police record and money.
.22 Long Rifle is a longer cartridge.
22 bullets come in short, standard and long. Your rifle is telling you that you have to purchase .22 long shell bullets.
The length- short, then long, then long rifle- also the power. Long rifle usually has a heavier bullet.
22 shorts can be fired in a pump or bolt action rifle chambered for long rifle cartridges but not in most automatics.
the 22 long rifle holds 11 cartridges
.22 Long Rifle bullets...
You can request a owner's manual at the Ruger website.
it means you can use 22 short rounds 22 long rounds and the modern 22 long rifle rounds and it will shoot them all
22 Short, Long and Long Rifle differ in the case length and/or amount of powder/bullet weight.
It means the firearm can fire .22 Short, .22 Long, or .22 Long Rifle cartridges.
Ask a gunsmith for help