Hey man!
I'm by no means an expert on Melee, Jigglypuff, or the Marth matchup at all, however I have played it enough times to throw a few tips your way. First of all I'd check out this video:
https://www.youtube.com/watch?v=h7IkCDl9Fqg It's not that in depth but touches on the basics of neutral and edgeguarding Marth.
I'd also add that there are a lot of little things you can do to help yourself out a ton in the neutral (note that I really don't know specifically what you're struggling with so this may be a bit general). If he's going for a lot of grabs or dash attacks CC rest punish on either of those is pretty darn easy and good for a cheap kill (you can also duck under his up-tilt and rest punish. A lot of Marths like up-tilt as a kill on jiggs so if he's getting greedy hurt him for it). It's also good to know what attacks you can punish with a rest OOS. Poorly spaced f-smash, d-smash, and dash attack are all easy rest punishes OOS.
Other than that the best way to figure this stuff out is just to play the matchup as much as possible. Learning the spacing on his aerials is super important and I find most of the percent I rack up (especially that crucial early 30ish percent) is from forcing him to whiff an aerial and tagging him with one of my own. Ideally you want to space yourself just barely outside his tipper range. This may suck when just starting out because you'll eat a lot of tipper f-smashes, but just keep at it and you'll get there soon.
I hope those few tips were helpful. There are a lot of good resources for this matchup out there, but I just wanted to share the basic things that at least have worked for me in the past.
