The King’s Indian Attack is more an opening system then an opening per se,
it involves the moves 1. e4, 2. ♘f3, 3. d3, 4. g3, 5. ♗g2, 6. ♘bd2, and 7. 0-0
(although, as you will see later, I'm going to recommend a different move order starting with 1. ♘f3 below).
It can be played against any opening move that black makes, except one, the Scandinavian, which I will address below.
What this means for you is not having to learn all the different openings and their myriad variations.
Here is a game,, played by Bobby Fisher that illustrates some of the possibilities of this system. One of the interesting aspects of this game is that Myagmarsuren playing Black does everything right, does not make any bad moves, but still loses to Fisher.
Okay, now that I've just taken you through the classic King’s Indian Attack move order with 1. e4, I'd like to recommend another move order, which can reach the same KIA formation, starting with 1. ♘f3 and followed by g3, ♗g2, 0-0, d3, ♘bd2, and e4, call it the Réti King’s Indian Attack (it also avoids having to study the Scandinavian, because now if Black plays d5xe4 you can just take back d3xe4 and you now have an open file for your Queen's Rook). The advantage of 1. ♘f3 is its flexibility, it can easily transpose into any number of openings. This allows you to wait and see what Black does before committing to a particular plan, although I would recommend sticking with KIA plan as it limits Black's responses. Note: The most common reply by Black to 1. ♘f3 will be 1...♞f6 (Black openings often start by mirroring White's first move). Other popular replies are 1...d5 and 1...c5, in that order.