In the London System White starts first move 1.d4 following 2.Nf3 and 3.Bf4. Or play 2. Bf4 before Nf3 which is considered a bit better because you don’t obstruct the queens diagonal d1-h5 which might prove useful later on as it stops Nh5 attacking the bishop f4.
Be aware that there are three common Black setups. Study them all to get familiar with them. See below
Black answers d5
Black answers e6
Black answers g6
The London System can be dangerous for Black if White is well prepared. You can use it against somebody who expects that you play the usual 1.d4,c4 stuff as White.
You can replay chess games of this chess opening below. Recognize the games where Black answers d5, then the games where Black answers e6 and at last the games where Black fianchettoes his kingside bishop to g7.
Replay Chess Games of the London System
Flip Board: Press F-Key (or click e7 or d2 on top) Select a game: Click on grey bar