It depends upon the kind of game you prefer to play. If you like a closed game, you can start with d4. If you like an open game, play e4. There are variations, however, that can lead to the same situation. This is called transposition. If you are not familiar with the strength of your opponent, choose a closed game. Another criterion to decide whether to opt for a closed or open game is the type of your play. Basically, there are two types of play: positional and tactical. If you are a tactician, choose e4, which offers plenty of opportunities for a tactical battle. If you are a positional player like me, though, choose d4, which makes for a quieter development, preparing everything before commencing an attack. There you have it. I hope this helps.