Bicycle Tutor

I have a brand new beach cruiser bicycle and the rear sprocket is loose, therefore making a lot of noise when pedaling. Also of note, there is a random O-ring hanging from the drive side axle (see picture). Any advice?

(I'm new to the forum...sorry if this has been answered previously or if it's an extremely simple fix. I've never performed any kind of bike maintenance before)

Well if its a new bicycle my first suggestion is to take it back to the seller and have them fix it or exchange for another bike.

Otherwise if thats not possible. put the bike upside down unscrew the two nuts holding the wheel on take the wheel off and see what needs tightening to hold the sprocket in place.

Looks like a slotted ring is the keeper but cant tell much from picture . The steel ring looks like it needs to fit behind it.

As George says - it looks as if the ring (split-ring / circlip?)needs pushing back into the grove.
An easy job but shouldn't have come out in the first place.
Make sure the ring is properly in the grove and check the grove first for cleanliness.

Is this a bike with hub gears, if so, I would strongly recommend that you do not work on it yourself unless you know what you are doing, you could invalidate the warranty, if from a shop, return it, if possible without riding it again, if bought on line ring their customer services, they will probably tell you to get it fixed locally and send them the bill.