A couple of questions:
1. Do you know if it's a PowerPC Mac or an Intel Mac? If it's a PowerPC Mac, your only option for a binary distro (i.e. a distro where you won't have to compile all your applications from source) would be Debian itself, which isn't exactly newcomer friendly.
2. If it's an Intel Mac, do you know whether or not it's 32 bit or 64 bit? This is
incredibly important, because most Linux distributions are gradually phasing out support for i386/486/586/i686. Distros like Debian, Ubuntu, and LInux Mint still support 32-bit Intel/AMD processors, but they explicitly require PAE support to be enabled in the BIOS.
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Assuming you have a 64-bit Intel Mac, I'd probably recommend
Rocky Linux if you want a stable, well-tested, and ultimately
incredibly long lasting distro. It's a rebuild of Red Hat Enterprise Linux, which with some
tweaks becomes quite a robust desktop system (I know the tutorial I linked is technically for CentOS, but 99% of the tweaks should still apply to Rocky Linux as well because Rocky Linux is made by the same developers as CentOS).
If you have a 32-bit Mac, I'd probably recommend
Linux Mint Debian Edition. It's 100% binary compatible with Debian as a distro, but it's nowhere near as obtuse as Debian itself is. It basically does the Linux Mint shtick to Debian where it makes the entire operating system as accessible to newcomers as possible.