Learn Verilog and practice implementing logic on a FPGA. A MicroZed board (Xilinx Zynq) is a great choice. You can even implement your own CPU.
Or an Arty! [1] It's got an Arduino shield connector and super affordable.
[1] https://www.xilinx.com/products/boards-and-kits/arty.html
Which FPGA vendor is most "Linux-friendly"?
TinyFPGA BX is pretty good (all open source tooling), but generally everyone has linux tools. edit: unless you meant which one would boot linux best :P Answer is "the expensive ones"
Ha, yes I meant tooling. But what if you want to scale your design to a different platform, would the open source tools still work?
For synthesis, the only FPGAs families currently supported by an open source flow are the Lattice iCE40 and ECP5 [0]. The latter is something you can be decently productive with and can fit quite a bit of logic (think: Amiga reimplementation, PCIe interfacing, etc.).
If you'd like to port synthesizeable code from the open source world to the commercial world, this _should_ just work as long as you're willing to rewrite any physical interfacing code (since those depend on hardware blocks available in a particular family) and stick to high-quality Verilog. But that's the same as porting across any other FPGA families.
Disclaimer: I work with SymbioticEDA, who develop and provide commercial support for some open source digital logic tooling, like Yosys and Nextpnr.