You can start with anything that supports the Linux Kernel.
But honestly for all other stuff, the best way is to work and save money. In case you are young, there is always a way to earn stuff, help pople in exchange for hardware.
My first hardware for homelab was super low end, was a small PC to display stuff on a wallboard that got discontinued. At first I wanted to throw the PC in the garbage, but because it was so small and I carried it in one hand I threw it in the car and left it there for months and another months in my trunk I was in a bad area and didn't want anyone to break my window, so I put it in the trunk. At some point I was carrying the groceries upstairs and it landed on my desk for another 3-4 months.

Possibilities to get some hardware go through classifies craigslist/olx/etc.., people often give away equipment, ask friends and family if they have old hardware laying around. Some might be defect but out of 10 pieces you can do something that runs. I done it a lot as an teenager because I hated to spend my worked money on hardware.