http://smashboards.com/showthread.php?t=42749
Use that for reference for anything you don't understand in my critique. Just use ctrl+F to skip to whatever you need.
First, I want to say that you should never hold down when Peach is down smashing. In fact, you should hold up instead, so you only get hit by the first one.
The first technique I suggest you practice is the SHFFL. Shorthop, Fast Fall, L-cancel. This technique is essentialy just a dramatic speed boost. You should also focus on your aerial game much more than your ground game. Short hopping will help the most here because you'll be able to do aerials at gound level.
After learning how to SHFFL, I suggest working on your spacing. A lot. Try to land hits at the very tip of your sword. Not only is your sword more powerful there, but it also acts as a barrier so that your opponent can't actually reach you. Tipped attacks also have different properties than untipped attacks, so it's kinda like Marth has two movesets that you have to master and then mix together. This is one of the harder aspects of Marth's game, and takes the most practicing.
Then you need to learn what to do on the ground. Dash dancing is possibly Marth's most important movement technique. You basically flick the control stick back and forth, so that Marth can go in and out of your opponents range. It's great for baits and when the opponent catches on, you can just switch back to actually following though after a dash rather than just dashing back out.
Wavedashing is another thing that's pretty helpfull to Marth, and you should find time to learn it. Wavedashing is one of the easier techniques to learn as well. Simply start by jumping and airdodging straight down so that you hit the ground. Then try doing the airdodge right after the jump. Practice doing the airdodge earlier and earlier, until you can do it without actually leaving the ground. Then, do the same thing, but angle the control stick between down and horizontal. The more horizontal you get it, the further your wavedash goes.
Before you work on actually fighting, you must master how to move Marth around and position him in the spots where his attacks will be most effective. You must also learn the flow of Marth's attacks. "How quickly can you move after doing this? Am I really making my next move as fast as possible?"
That's all the advice I'm giving for now. I may give more later. I hope this isn't too much or something.
