Some experienced positional players I know, who don't have the time to learn lots of opening theory, play first move Nf3 (then g3, Bg2, and d3) and go for the Kings Indian with White, which is called The Kings Indian Attack.
The Kings Indian Attack ( or Barcza System ) is a chess opening setup for White and not a forced line of moves. It can be used against different Black openings such as the Sicilian Defence (1.e4 c5) or the French Opening (1.e4 e6).
See this solid setup of the black pieces above that gives Black a comfortable game, because the power of the white bishop-g2 is reduced by the black pawn structure d5,c6. You can say the white bishop bites on granite. Black should play h6 to be able to retreat with the bishop to h7, when it gets attacked. Black should start counterplay at the queenside.
When you have started with 1.e4 you just follow by d3, Nd2, Ngf3, g3, Bg2, and 0-0. Or just play a different move order and begin with 1.g3, 1.Nf3, or even 1. d3. This closed system is very flexible and is suitable for strategic players who prefer to accumulate small positional advantages over time.
But don't be mislead by its name as this opening is not necessarily an attack. Don't try to overrun your opponent as Black can reach a comfortable position easily and you cannot expect to have a small edge as White that you should have normally playing the white pieces.
This is the reason why the King's Indian Attack is usually not used at the highest chess level but is played more on the club level.
Furthermore the game will often evolve differently from that of a typical King's Indian Defence played by Black.
The set up is easy to learn but if you are a beginner and have no understanding of positional chess you should not play this positional opening but rather play open tactical-rich games starting with 1.e4 like the Italian Game or the Ruy Lopez (Spanish).