I think the natural first step would be to learn Bash:
When you learn, you're going to encounter tools like:
grep, sed, awk, find
You'll find LOTS of tutorials online on those tools.
When it comes to sed, this one is one of the best:
It's a lot of information to take so start from the basic stuff. Try doing some projects, eg. to automate something on your system.
Ideally, when playing with it you should back up your data or do it on a testing system as you might mess something up or delete some files.