How do you adjust chain
Flip bicycle up-side-down so it sits on the seat and handlebars. Loosen the 2 nuts that hold the rear wheel on. Sitting behind the bike on the ground, so that you can align the wheel to a straight position, pull the wheel back so that the chain is snug. You don't need to pull hard, just hold the wheel in place and straight. Using the other hand, slowly tighten the axle nuts back down until the wheel is held in place. Once the nuts are tight enough to securely hold the wheel, tighten the nuts. Sometimes this method will make the chain too tight. If this happens, repeat the process as above, only this time when you go to tighten the nuts back down, squeeze the chain slightly to create a bit of slack. Ideally, the chain will be tight enough not to skip or slip, and at the same time, loose enough not to make noise, bind up, or do any damage.
There are "chain tensioners" available that hook around the rear dropouts of the frame and also around the axle, allowing you to tighten the chain to exact specs. simply by tightening or loosening adjuster screws. They will also help to keep things from loosening up over time. These make things much easier and I highly suggest looking into them.
Use any of a variety of oils and lubes available, but try to avoid WD-40 for chain lube. It will stop a squeak for now and make things seem to move smoother, but in the long run, WD-40 will attract dirt and debris to the constantly moving parts of your chain and can shorten the life of a chain considerably. Hope this helps.