Since I have never played Melee and had a hard time adjusting to P:M I feel like I should have a few good pointers, but I don't. So I'll just say a few things, some that helped me personally, and some that I think are useful to all players;
1. Don't be afraid to try out all characters. This is pretty much a given, but I feel it's more important in Project M, due to the attempt to balance the characters out. Unfortunately, there may be some Brawl favourites that were changed to have a new style that you are uncomfortable with, or perhaps you might just find some of these new styles to be fun and interesting so you might pick up characters that you didn't use before! Play who you want.
2. Persistence. Self explanatory.
3. L-Cancelling is as simple as pressing a Shield button when you hit the ground whilst performing an aerial attack. You'll flash white when done correctly. You said you had trouble comboing, well this is a vital technique in expanding and perfecting your combo game. Basically is allows you to end your aerials quicker, so you can input another action faster without being punished as easily, if at all. This can be another attack, movement, shield, etc.
4. Air dodges defy gravity in Project M and can only be done once before landing. They can be angled to have you dodge in different directions, which leads us to
5. Wavedashing is doing a short hop (pressing a jump button softly or tapping on your joystick upwards quickly if you use tap jump) and using your air dodge towards the ground at an angle. Speed and precision is key here. When performed correctly, your character will duck and burst forward or backward. Every character's wavedash is different. So some are more useful than others and some are easier to do than others.
6. Camping and stalling is nowhere near as prominent in Project: M as it was in Brawl. Unless necessary, try to stay on the offensive.
This last one is very much a personal thing but I'm going to say it anyway because it really helped me get into a better mindset and improve my skills...
7. Keep your mind on your opponent, not yourself. If you're thinking "I should do this move now" chances are, your opponent is reading you. Falling into habits and pre-conceived ideas of what you should be doing is a bad thing in any Smash game, but is worse in M than Brawl. In M, every character has so many more options than Brawl. This will improve your combos because rather than thinking "oh, now I have to do this move", you'll just assess what your opponent does and you'll instinctively use an attack that will hit them, rather than missing or at least becoming too predictable because you keep doing the same follow ups and starters. This is also good from a defensive perspective too, because as I said a few times already, you don't want your opponent to dodge or block every one of your moves because they know what you will do.
TL;DR Keep your mind open. Don't go on "autopilot".
