Read the Linux programming interface by Michael Kerrisk. It will tell you what Linux should be doing for you ( the api basically and what subsystems exist). Read lwn, kernel newbies to get a feel of what’s happening in the kernel.

Get knowledge of operating systems ( I like the minix book by tanenbaum ). Learn about Linux internals ( I like linux kernel development by Robert love)

Then, pick bits you’re interested in and jump !