At least half of the people who will respond to your question will just tell you to play their favorite game “X” where X=KOF 98, Garou, etc.
The only thing you can know is that XII is going to be significantly different from any other KOF, but learning the basis of any KOF will likely help you.
The solution is to try out the games that are generally considered the best: Garou, KOF 98/UM, KOF 2002/UM, and KOF XI. Decide which of those you like the best, and then just play them.
Fortunately, it’s not that expensive or hard to try KOF.
Garou, KOF 98, KOF 2002 are all available on GGPO and 2df and have communities of good players to help you.
KOF 98 UM and KOF XI are available for US PS2 and very cheap ($15-20 for KOF 98 UM, $10-15 for KOF XI).
The only exception is KOF 2002 UM, which is right now only available via Japanese Import and runs at least $50.