So I started playing about a month and half ago. I play with some school friends on dolphin. I can wavedash consistently and still learning to shffl. What should I learn next as a marth main? Should I have a secondary? Is buying a crt for my wii worth it? Should I enter resistance 4?
don't worry about wavedashing. if you can't shffl consistently, you won't be implementing wd as well as it could/should be used. if you can, drill out shffling in spare time (if you can't play on the setup by yourself for long enough to practice, just make that your primary goal in games, even at the expense of winning).
and work on learning to implement proper dash dance usage against people. the video meebius linked is one of the better summaries of why you dash dance and how to use it in a match. i'd even say go so far as to basically make your marth consist entirely of dash dance, grab, fair, and fsmash for a while (ok, maybe not the best idea, but that's the basic moves you build the marth neutral and punish off of, so they're good focuses).
once you've gotten the hang of dash dancing in neutral and how to bait, react, and punish, then start using wd more, since that's its best utilization (moving some set distance while maintaining your facing).