If. you are new to After Effects spend a half-hour in the Learn workspace walking through the UI and the basic workflow. Then consult the User Guide. It is always available on the landing page for this forum or on the Adobe After Effects Product page. Trying to just jump in and make something up almost never works for anyone.
When you start messing with the Composition Settings and stray away from the included presets, you are asking for trouble if you don't have a good understanding of video formats, video standards, and even color. All videos rendered for sharing or playing with a standard media player must fit into a specific frame size/frame rate/format or they will get fouled up.
When you have problems with a project please post a screenshot using the toolbar above the reply field to embed the images.*
Make sure you select the problem layers and press the U key twice to show us what you have changed. If you are having problems with composition settings at least have the comp selected in the Project Panel so we can see what the settings are.
I hope this helps you get started. If the comp has the same horizontal and vertical dimensions but the comp does not look like a square then you have set the Pixel Aspect ratio to something other than square. Non-square pixels are not used at all anymore unless you are rendering for an anamorphic output. Only the big kids with the big budgets and the really big cameras can afford anamorphic lenses so you probably don't need any of those presets.
* The "Drag & Drop here..." area is buggy, a pain to use, and should not be used to share images. Please use the toolbar or just drag your images to the reply field so we can see them when we open the thread.