My stepson has a mongoose outer limit bike purchased at christmas. He has not ridden it alot, but here recently the rear sprocket started slipping when peddling and making a noise. I have put some spray lube on it and that did nothing. The teeth on the sprocket isn't worn, so I don't understand what could be causing this. Your help would be appreciated

Do you mean the chainwheel (front "sprocket") or the cog(s) (rear "sprocket"? If the former then crank bearing is out of adjustment, if the latter, then the freewheel/cassette may be bad or the hub bearings out of adjustment. As you can see there are too many possibilities to diagnose remotely. I would suggest you get the help of a bike co-op, a knowledgeable friend or a bike shop do determine the problem and fix.

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To remove any rear freewheel or cassette you will need to purchase a Park Tool. Park Tool is the best brand when it comes to bike repair. Google Park Tool and you can look through their products. You will need a cassette/freewheel remover. There are dozens of different companies that make hubs and cassettes (gear sprocket). Take a look at yours and see what company made it. You can than find out which tool with properly fit your cassette. These tools are made to fit in a wrench making hand removal easy. Some brands will require a chain whip. The only way you will know which too you need is by find the company name on the cassette/hub. Without this tool removal is virtually impossible. If you need help narrowing down which cassette remover you need you can always let me know the company name and I will let you know the proper cassette tool to purchase.

Any Mongoose dealer or bicycle shop should be able to handle that or just web search for a 20" BMX wheel. It doesn't appear to be unique.

if i remember correctly it is a 15 mm socket for a ratchet that is the tool that i use at the shop

Derailleur. Everything on a bike that bolts on can be replaced. Take it to a bicycle shop for assessment. Special tools are involved.

Try and move the whole wheel back a little and see if that helps. if not adjust the brake pads untiill the contact the rim and not the tire

Local bike shop determined that both wheels were defective. I took the junk back to Walmart and got a refund... PROBLEM SOLVED!!

try buying a new back wheel or sprocket?

Theres a special wrench you need that fits the gear. Its easiest to take the wheel to a shop that has one and they will also be able to replace it with one with the teeth ratio you'll want.

A special sprocket removal tool is required usually only $20 to $30 bucks at a decent bike shop.