How do I determine how much data I have to Back up on my MacBook Pro?

If you are only concerned with estimating the size of your *data* then a good basic measure would be to look at the size of your user home folder. (Do GetInfo, aka CMD-I, on your user home folder; in the resulting window, look at "Size." Of course this assumes that you only save data in folders inside your user home folder.

OTOH if you want to back up your entire drive, then do GetInfo on "Macintosh HD" and look at "Used."

But as @MrHoffman suggested earlier, if the purpose is to estimate the capacity needed on a backup drive, figure on at least 2x-3x the size of the data you want to back up so you have room for multiple backups plus some headroom.

Usually two to three times the storage capacity of your internal storage for the external hard disk drive, or potentially more if you want more depth of backups.

You can go with smaller external backup devices, but Time Machine doesn’t do well with smaller storage capacities and lower ratios, as the overhead of the churning increases. And internal storage usage tends to accrue.

Apple > About This Mac displays, and the System Information display shows storage capacity, as well as usage.

If you want to create a Time Machine backup on an external drive, you can configure which data you want to back up in the settings for Time Machine. As you select or deselect the data, Time Machine will show you the actual needed size of the backup.

Exclude files from a Time Machine backup on Mac - Apple Support

Back up your Mac with Time Machine - Apple Support

Do you store any important & unique data on external drives as well? Then you need to add that data into the computations for your TM drive as well assuming you will also want to use TM to backup your external data drive(s).