This is such a broad topic, its impossible to give an advice or place that works for everyone. What I would do is pick up a few common commands that are not destructive in their nature (no need to practice with rm command in example) and read its manual man grep and help grep --help . Then try to understand and play around with the options and dedicated test files. Search tutorials and tips for these specific commands you are learning about. Over time this should result in deeper understanding of various concepts, as as you learn about grep, you will also learn about pipes and files in general while you are researching.
Then you can move on to other commands. Maybe setup a virtual machine where you can experiment a bit more freely, but that's probably overkill. Also look what people do in their Bash scripts, as Bash is contains the commands you can use on the terminal too.