Advice from a long time Mac user:
1. Get Steam. Lots of Mac-friendly games there
2. Buy anything Blizzard makes; it's all Mac compatible. Note however that Diablo II (and presumably I) are no longer supported by the newest Mac OS
3. Buy Torchlight, excellent Diablo clone for 20 bucks or less now
4. Try Boot Camp, if indeed it still exists. I used Boot Camp 5 or 6 years ago to run all my PC games on a Mac with no problem
5. Get a console, because Macs are limited for gaming