OH and one more question. If someone was to not respect my space/range what can I do to tell them to "back off" I guess. Is it simply bait out approaches. But what if I can't gain any space and they're constantly in my face?
If you can predict when they're coming in, D-tilt beats grounded options, SHFF fair and nair in place beat aerial options, and dash dance grab will beat anything while being a bit riskier.
However, when someone is constantly in your face, that generally means that they've beaten your spacing. This is something that typically happens if you're trapped at or near the ledge of the stage, stuck in shield, or stuck on a platform. The solution to this is not just "Throw out a move and hit the opponent". Sure, if they over-extend and actually do let you hit them, that's great, but typically you'll only offer them a free punish.
Generally speaking, your goal is to get back to center stage, and get enough room to start up your dash dance again. There's no single good option that solves Marth's problems here, so it's a matter of knowing all your options and which is the least risky at the time.
If you're seriously overwhelmed and you need a moment to think, you can pull a M2K and retreat to the ledge. You're still in a bad position, but you have time to stall on the ledge and think up a gameplan. (Just be wary of doing this against Fox if you can't do the invincible waveland stall, as he will just run out and shine you.)
Sometimes you can jump and retreat to the platforms. Plenty of characters can threaten the lower platforms pretty easily, but there aren't that many that can safely and effectively pressure the top platform, especially if you know how to shield drop. Another mixup you can use when you jump to a platform is to immediately waveland back off the platform, potentially coming down with a fair or a bair.
If they're being predictable about when they're coming in, you can try a counter. Falco is the only character I'd really recommend this option against (considering how he always has hitboxes out and his grab game isn't as terrifying as a Fox or Falcon), but it can work against others if you know they're throwing something out.
One thing I like to do is to straight up run past them, and then use my wavedash to reset into my dashdance. While Marth doesn't have any moves that are amazing at controlling the space right next to him, one of your advantages is that you're an incredibly fast character. Unless they expect it and react in time to hit you as you initially dash through them (which they shouldn't be trying if you've made them respect your down tilt and dashdance grab), it's extremely difficult to cover all of Marth's movement options, simply due to your speed.
Finally, if they're in your face with shield pressure... you're kind of screwed. Marth doesn't have any out of shield options that will break through Fox or Falco's pressure, and once they're already on top of your shield, you're pretty much another casualty of 20XX.