A HDL looks like software (and is obviously) but it is an abstraction of hardware. You have to understand digital logic and the relationship between a HDL construct and the digital hardware. For example, an if-then-else tree has a different implementation than a case statement. While they seem to do the same muxing, the first one has priority built in, the second one has not. Hence, study digital logic, learn HDL (verilog, VHDL, …) and buy a small FPGA development board (less than 100 EUR/USD) and start experimenting.
What is the best way to learn FPGA professionally?