In the London System White starts first move 1.d4 following 2.Nf3 and 3.Bf4.
Be aware that there are three common Black setups. Study them all to get familiar with them. See below
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. The games are replayable in the order as shown above. This means first you find 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.