How do I upgrade my old computer tower?

Hello there, I'm curious about how I can upgrade my old computer tower. It's been a while since I've last touched it and I'm feeling like it's starting to show its age. What steps should I take to give it a much-needed boost in performance? Should I start by upgrading the RAM, the processor, or perhaps the graphics card? Also, do I need to worry about compatibility issues when choosing new components? I'm not too tech-savvy, so any advice would be greatly appreciated. Thanks in advance!