Your comp is set to Rectangular pixels. Until you become an expert in video standards and formats you need to stick with the default composition and rendering settings. Change the Comp Settings and make sure that the rectangle shape is square by revealing the property settings on the shape layer. Do that by pressing 'uu' and spinning things down.
Be very careful about the tutorials you choose. Most of the new tutorials on YouTube are prepared by amateurs and enthusiasts and most of them are pretty awful. Poor workflows, incomplete explanations, and really bad advice given by well-meaning enthusiasts have given a lot of people that ask questions on this forum some real problems. I spend way too much time helping folks work through problems caused by poor tutorials than I should. Make sure you vet your trainers. The number of views won't tell you if the tutorial is any good. I helped somebody with a tutorial the other day that had more than 100,000 views and the technique shown will only work if your footage very closely matches the presenter's footage.
If you are new to AE, open up the program, and choose the Learn workspace. Spend 20 minutes going through the UI and the first tutorial. Then open up the User Guide and start exploring there. A couple of days on the basics will save you weeks of fiddling with workflows that are poorly thought out and poorly explained.