guide to correctly assemble a Walmart bike
If you are mechanically inclined most of the adjustments are straight forward. There are many on line tutorials, Park Tools website and the often mentioned Sheldon Brown are my personal favorites. As @ Batman has stated BSO (bicycle shaped object) can be frustrating to maintain. I have serviced them and the bearings don't get any smoother when I assemble it than it did from the factory. The primary concern from the design is will it function (not is it smooth ,is it light ,is it durable)  with parts from a certain pricepoint To use a comparison you may relate to. If I buy a tablesaw for $125 it may last me 20 years of occasional homeowner use, if you used it commercially it will fail much sooner. A Walmart grade bicycle will function fine for an occasional ride before it needs maintenance. A higher quality bike (not just when new but after years of use) will be easier to tune and stay in tune longer. It will be repairable and upgradable.