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.
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. 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.