Always open with the Queen's Pawn versus the King's Pawn. It is a much stronger position.
Most of the remaining opening moves only work against players who are not familiar with countering them. I've never liked the Queen's Gambit (pawn to D4+C4 openings) against an experienced opponent. Queen's Gambit does bugger up a weak opponent for a while, however it leaves white's front row exposed. Once the black has chance to play for position and to gain the initiative, then the D4+C4 opening leaves white vulnerable. In particular, if the moves go Queen's Pawn to D4, Queen's Pawn to D5, Queen's Bishop's Pawn to C4, King's Knight to F6, C4xD5, F6xD5, then black effectively has the initiative. Countering with King's Pawn to E4 is particularly ineffective, because White's queen is protecting D4. With with no pawn's for cover, white's queen is open to attack, and the threat of the attack can often be used to gain at a one pawn advantage (or a significant position advantage).
To break 1500 you should review all the key opening moves, and how to counter them. It will not be as easy to bluff a 1500 player with strange opening moves, and they may use some of these unorthodox openings against you. Be on the lookout for Queen's Gambit Reversed - a popular counter against an unusual white opening.