You'll have to spend time in training mode or something to practice movement (dash dancing, wavedashing, wavelanding) and then you'll have to practice that movement in a match situation (totally different). During a match try to do a wavedash back to get out of pressure, or in between stocks practice wavelanding on platforms. Up tilt is super good. Practice hitting the different parts of up tilt seeing what the knockback trajectory is (don't forget about the hitbox behind marth).
For the falco MU:
Falco has a way better approach with his lasers. You can dash under some of the higher lasers I believe, counter them if the falco mindlessly jumps into you, or move around platform (if available) to get away from the lasers and try to approach yourself. Falco's recovery is awful, so a falco off stage should be dead, or close to dead. Marth's recovery without a jump is bad too, so you'll probably die too. Think about how Falco is going to edgeguard. Marth has the luxury of being able to gimp Falco easily with side b's or bair/fair etc away from the stage (if they up b). Falco can grab ledge to hog Marth's up b or ledgedrop bairs. However, you can sometimes tech Falco's edgeguards from on stage. Dair and d smash are common-ish Falco edgeguard and you can tech them I think sometimes. As Marth against Falco you want to avoid being stuck in shield from Falco's lasers. If you do get stuck, wavedash out of shield is a good option (you can up smash and up b out of shield, up b probably being the better option, but I have yet to really try up smash out of shield, I don't think the knockback is good enough). If the Falco is being smart, they will pressure your shield outside of your grab range (like behind you), but if they mess up a jc shine or something in front of you you could try a shieldgrab. Falco on a platform like on yoshi's will take a bunch of damage from up tilts, up airs, waveland grabs, etc.
Summary:
These are just my notes, I'm still a noob Marth so I would listen to the other guys on here
Edgeguarding Falco:
If they up b - side b's, fair/bair, counter by the edge
If they side b - d tilts or jab (+ jab 2 if you want to cover smash DI) - > another d tilt then they probably have to up b so refer to the first bullet
Don't forget about grabbing ledge. Grabbing ledge is so good (wavedash off FF, bair to ledge grab, moonwalk)
Try to avoid getting stuck in shield as Marth is limited out of shield unlike the spacies where they can shine or up smash (more fox though)
Dash dancing/platform movement is super powerful spacing tool against Falco (he's slow, so if he can't laser you it's tough for him)
DI - combo DI out of Falco's combos as early as possible, Marth is floaty enough to get out earlier than others.
Grab on Falco = lots of percent
Up air/up tilts beat up on Falco pretty good
Falco's dair is insane, but you have a spike too (NTSC) and you have a tippered f smash which wrecks Falco and his sub-par recovery.
Hope that helps