I have a MacBook Pro 15" 2012. I need to update my macOS so that I can run the new Lightroom. HELP!

A mid 2012 through early 2013 MacBook Pro can be upgraded to Catalina 10.15.7 but no later. See: How to download macOS - Apple Support

It isn't possible to upgrade it to Big Sur 11.0 or later (the minimum for the latest Lightroom).

Be aware that if you upgrade to Catalina you will also need to update any non-Apple 32-bit apps. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

I don't know what macOS version Lightroom requires.

Further to what FoxFifth wrote,

There are multiple versions of Lightroom available from Adobe.

• Lightroom Classic 10 can be installed on Mojave (macOS 10.14) through Big Sur (macOS 11.x)
• Lightroom Classic 11 requires macOS Catalina (10.15) to Monterey (12.x)
• Lightroom CC requires macOS Big Sur (11.x) or later.