To print from your home printer, you can just use File>Print in whatever the native format of your software is.
To send it out to a pro photo printer, you'll usually use File>Save AS and choose JPEG, and have NO compression, or HIGHEST quality. Do NOT resize it first.
Typically, you have 3 copies
1. native layered format for editing
2. small JPEG for sharing online
3. large JPEG for pro photo printers
The recommended resolution for quality prints is 150-600ppi, with 200-300 being the most popular.
To print a square layout, use Image>Resize Canvas and make the canvas the standard rectangular photo size, then save that as a JPEG. The extra area you can either fill with something else, like a small photo or small layout, or just trim it off after getting the print. Costco now offers 12x12 and 8x8 prints in response to the digiscrapping community.
SELLING MY SIGMA NIKON F 10-20mm f/4-5.6 ULTRA WIDE-ANGLE LENS