Often times when playing Marth, you want your opponent to come to you, and you don't necessarily want to be the one approaching, so just think about using Marth's greatest strengths when you're in neutral situations-- His fantastic range (on both his grab and attacks) and his great dash dance and wave dash (you'll see that spacing is the name of the game with Marth-- It's really the name of the game in general, but Marth REALLY emphasizes this considering the composition of his character.)
For this reason, just try to really get familiar with just how far your attacks reach, understand this kind of threatening "bubble" you have around yourself when you're standing and have access to all of your tilts, smashes, jab, grab, etc. (or in the air with all of your aerials), and play around with your dash dance, how far you can go with it at its maximum length, how you can shorten it, how you can move around the stage while still doing it, and think about the advantage it gives you in your ability to fake your opponent out, cause him to throw out an attack, instantly dash back in and punish.
There really isn't a flowchart to Marth in terms of approaches, neutral game, and so on, and the game in general rarely has "flowchart situations," which is the beauty of it because it leaves so much room for creativity, so try to get less into the mindset of "what's my best option" and more into the mindset of "what's my best option
in this situation?" or even "what
might be my best option?"
As for the spammy opponents, it's something that all new players struggle with (I still get hit by stupid stuff when playing against friends who aren't too much into the game), and it's something you'll be able to deal with once you start understanding the concepts of spacing, adaptation mid-match, etc.
Before you know it, you'll be JV 5-ing those scrubs
Anyway, sorry if this is such a damn essay (I'm pretty bored right now lol), and hope it was at least helpful!
