When I play a card with haste, do I have to attack or use an ability, or can I just play it without attacking?
No, haste gives you the choice to attack or activate abilities with the tap (or untap) symbol in the cost, but does not create a requirement. If a creature is required to attack it will be explicitly spelled out like with Juggernaut. (As far as I know there are no cards that require you to activate an ability)
702.10b If a creature has haste, it can attack even if it hasn’t been controlled by its controller continuously since his or her most recent turn began.
702.10c If a creature has haste, its controller can activate its activated abilities whose cost includes the tap symbol or the untap symbol even if that creature hasn’t been controlled by that player continuously since his or her most recent turn began.
No, having haste doesn't mean the creature has to attack. Especially if you cast your creature during the second main phase: it couldn't attack that turn, period.