Beginners often ask about what opening move is best! It is like asking which of your legs should be put forward first when you start a walk! But from general principle of controlling the center, you should choose one of the four central pawns namely KP, QP, KBP, and QBP. Your choice depends on the style of play you like (more strategic or more tactical), the types of openings that go with it and so the first pawn move that is required by that opening theory! But since transposition is possible quite often, the first pawn move need not be a rigid requirement even for the opening strategy adopted by you.
In this respect, you may like to know opinions of some of the past masters. Emanuel Lasker, a world champion and one of the greatest chess players of all time, in talking about common sense in chess forbade using any opening move other than d4 and e4!
It seems that Paul Morphy, another all-time great and a chess genius, never used d4. On the other hand, Gruenfeld who was a GM and authority on opening play, used e4 only once in his tournament career (against Mark Taimanov in 1950 and lost the game)! When asked about the reason for not using e4, he stated that he never made a mistake in the opening! Probably once was enough!