Fastest way to FI: PPL, build 200hrs TT, CPL exams, FI rating.
Fastest way to FI so you can be paid: PPL, Night, hour build, CPL exams, CPL, FI
Most common way to FI with airline career in mind: PPL, Night, hour build, ATPL exams, CPL, FI. Then make damn sure you get your ME/IR done and issued before your exams expire!
If you don't want to be an airline pilot, DO NOT bother with the ATPL exams. It's extra work for no benefit.