Second play through in Majora’s Mask 3DS like there was in Ocarina of Time 3DS

On the Final Day, you'll find out what happens after your 72 hours are over. Given how early you are in the game, it's a bit spoiler-y. I'd recommend that you just play through to the Final Day and you'll see what happens after the three days are over. If you can't wait, the next bit is in spoiler text (just mouseover to reveal).

On the Final Day, the Clock Tower will open up. If you have access to the Town Deku's flower (through trading him the Moon Tear), you can fly up to the Clock Tower entrance and run up to face the Skull Kid. After a cutscene, you'll need to re-acquire your ocarina and you will prompted to play the Song of Time. This will rewind the timeline 3 days and thus the major mechanic of Majora's Mask will be revealed. You'll be constantly rewinding time throughout the game, losing most items and resetting progress with the exception of your masks, Heart Pieces, and key items like your Bow or your ocarina.

If you're asking about what happens you aren't prepared at the end of your first Final Day, I hope it's not very spoilery to say rocks the Moon falls and everyone dies you get brought back to the Dawn of the First Day.

As a person who HAS Majora's mask AND OOT, I can say the controls are the same. The items are slightly different, though. You WILL learn fast, it is very similar to OOT.