I'm brand new to selling my photos, I'm having a blast taking youth sports pics. Is there a way I can take the cropping guesswork out of my customer's hands and do it for them before I send them to their gallery?
You can but if you crop before uploading, then ONLY sell sizes that are compatible with the crop ratio you've chosen.
Generally it's much better to upload the photo uncropped, and allow the customer to crop in the cart, or even better, you can crop in the Proof Delay screen for the customer. You can also recrop an order that the customer may not have cropped perfectly. I generally shoot with plenty of cropping room for customer-popular sizes, such as 8x10 and 5x7. Don't worry about resolution, today's cameras can handle it!