Practice movement. A lot. Smash 4 is cool and I love it, but it just doesn't have the same wealth of movement options as PM does. So go into training mode on Battlefield and just grind out dash dances and wavedashes/lands and ledgehops and all the rest until it becomes second nature, just a part of the way you move around the stage without really intending to. That'll be a huge part of being good at a game with Melee-style physics.
Learn about the characters. Some people say Project M is more matchup-dependent than Melee, which I don't believe to be true—you just need to realistically learn more than like eight matchups—but it doesn't change the fact that there are more matchups to learn. The cool part of this bit of advice is that—at least in my opinion—it's a lot of fun. There are so many cool tools and tricks that PM gives to a huge variety of characters, and learning what they can do in relation to your main is not only useful but also pretty interesting imo.
Speaking of mains, the question of 'who should I main?' is always a hard one to answer, but it is especially so in PM. The great thing about the game is pretty much everyone is viable. It seems like you have a history of playing Luigi; he's quite good in this game. Decent recovery, weird movement options, a good combo game and damage output. But experiment. See who feels the most natural fit for you. Learn a bit about some of the characters' unique abilities until you find one that makes you say "Ah, that sounds cool!" and try to do it. Get familiarized with the different properties of each attack. Smash 4 recycles move uses a bit (most down throws are the combo throws, most up airs are the juggle moves, most back airs are 'shove the opponent away from the stage' attacks), but in PM there's quite a bit of variety. Some characters are totally unusual—Lucario, Snake. See who clicks. Best of luck with PM! It's a really enjoyable game.