e4 is played by White to develop his pieces rapidly and castle. e4 is a good move for beginners! You learn to develop quickly and to attack, since it opens up the Queen and Bishop. If met by e5, you have what's called an "open game." Then the most popular second move is Nf3, though other 2nd moves are fully playable: d4; f4; Bc4; and Nc3.
If Black does not answer with e5, you have a "semi-open game." The most popular semi-open defences start with 1. ... c5 (Sicilian); e6 (French); c6 (Caro-Kann); d6 (Pirc); g6 (Modern); Nf6 (Aljechin); Nc6 (Nimzovitch); d5 (Scandinavian); and b6 (Owens).