So I might as well repost the giant guide I made a while back, hopefully it helps. :D
Charizard can be played as a passive aggressive or full agro character. You'll win a lot of matches by spacing people with Nair and using it as your primary focus for the neutral game. Keep in mind that Nair is your best move, incredible spacing potential, amazing edge guard opportunities, and event good combo potential to boot.
In the neutral game you will usually want to dash dance around mixed with doing Reverse Arial Rush Nairs to outspace and pester your opponents shield. Try and keep you back to them as much as possible since that's where you have the most range and are the safest honestly. Keep in mind that Nair with destroy projectiles that aren't lasers or physical objects like bananas or turnips. However a decent alternative if you don't have you back turned to them is just to jab, the disjoint in big enough that it will still destroy projectiles, and also acts as an anti-air (it's almost a perfect counter to Mario's full hop fire ball approach since it clashes with the fireball and also hits him). If your opponent is ever above you, best option is to try and get under them and Usmash or even jab since it's got some pretty massive disjoints. Using this strategy is a little more useful against floaty characters like Peach and Wario, but just Nair camp against Puff. Also crouch canceling a jab out of a run is pretty amazing too, spaced properly it's pretty safe too. That's generally the basics of the neutral game, also get grabs if you can since you have a really good grab range. But don't get to greedy with it, you want to force them to make a mistake, not read everything they do.
The offensive game has a few different ways it can go.
First of all throws:
If you get a grab, you have a few options. Bthrow can equal a free Fair or a free sourspot fair to Dair (the Zard Ken combo), but this is only if they do not DI away from you.
Generally Dthrow is better because it's a tech chase that is weighed pretty heavily in your favor. When you Dthrow, look for a tech. If they don't tech, jab them, you get about 3 or 4 frames advantage if they don't tech. If they do tech you can do one of a few things, either chase for a regrab, or if you were predicting it, you can cover two tech options with a Dsmash properly spaced. This is usually best near a ledge cause you can cover pretty much every option. Keep in mind you can also jab if they tech in place but grab is a little bit safer. It's not quite so good you can just react, but you can get a lot of precent on reallllly quick. Also, if your opponent is at kill percent, just Uthrow. Even if it doesn't kill them, it puts them in a bad offstage position since you can super jump and pester them in the air.
Your combo game revolves around getting your opponent in the air, and Zard's got like 5 launchers so you should be pretty good at doing that. Jab, Dsmash, and Usmash are your best launchers (besides Nair I guess). Off of a jab you can get Fair or Usmash. Usmash doesn't usually directly lead into anything, but super jump chases with Uair or even a well spaced Nair are great. If you ever get super jump Uair, just chain 1 or two together then finish with an Up B at the top of the screen, it kills ridiculously early. Dsmash is very character, weight, and percent dependent. On some lower percents you can just straight Fsmash people out of it. at upper percents it leads more assuredly into the Uair chain Up B combo at the top of the screen. Dair functions in much the same way. However a lot of smash is figuring out the combos that work for you so feel free to experiment.
Where you can get a lot of leverage is your ledge pressure. A reverse Nair can easily hit below a ledge, nullifying perfect spots on any character. Fun fact it's also a semi spike. The timing can be strict, but it isn't too hard to learn. You also have Dtilt, which if you hit with the flame has about the same hit boxes at Marth's F smash, but not quite as below the ledge. Even side B can hit below the ledge. Also you have flamethrower, which can be brutal to a lot of characters. You can also stall the ledge by dropping off the ledge and gliding right back onto the ledge.
Your Defensive game:
You're fat, and you're really large, learn to accept this early and you'll do better. You can get juggled by falcon and MK like you wouldn't believe, expect it and DI you hardest to get out of that stuff.
Recovery: Glide is amazing, and glide attack has stupid priority, only disjoints can beat it. This should be your main recovery option for pretty much every situation, especially if you get sent high since you can glide high over your opponent, wavebouce the glide and Nair as you fall to cover yourself. Also remember you can cancel Glide into an up B, can be extremely useful in a lot of situations. If you get hit out of your glide though, you will loose all of your jumps, so don't get hit, go under the stage if you have to. If you do get hit out glide, don't forget you can boost yourself back with side b.
On the ledge:
You've got a few things you can do from the ledge. First of all you can glide camp by just dropping off the ledge and immediately gliding, done fast enough I think you can be fully invincible. You can fake them out by glide camping, but instead of camping the ledge just Glair, it will set you onstage and it's really wicked fast if people aren't expecting it (fun fact you can also waveland it if you're get the angle right, also works on platforms or the ground). You can also ledge drop double jump side b. If you do it correctly you should end up right back on the ledge so they can't punish it. You can also ledge drop double jump flamethrower. If you've got space to work with, it's a great way to give yourself a bit of room onstage. You can also do and normal ledge drop arial, Fair isn't bad, or you can be like falcon and Dair if they recover onstage. Or heck even Nair and you'll hit them with the semispike.
In shield:
You've got shield-grab for in front of you, short hop OOS Nair for cross ups, Usmash OOS for when they are above you, and Up B OOS for when you really just need to get your opponent away from you. Note that up B OOS is REALLY RISKY, but it has decent knock back, try to aim for a platform with it though so that you don't get in so much landing lag.
I would say that's the gist of playing Zard, I'd love to answer more of your questions.
If all else fails, just Nair a lot and you should be fine
