You should consider sifting through the Zard Video thread and watching some matches to get an idea of how some players like to use Zard. What I did when I started playing as Zard was mess around with his moves, try to get a feel for how he handles, and develop my own playstyle around that. Afterwards, I watched some Zard videos from different players and mixed in some of the tricks and move applications I saw them use.
Zard is a large, speedy, combo heavy character with some powerful hits and the speed to follow up. You'll probably find yourself popping your opponents up with jabs, throws (back and down in particular), downtilt and upsmash when your opponents are at low percent, and juggling them through the air with your heavy aerial mobility when their percents hit the middle. At high %, you have many tools and leads to finish the job from both the top and the side.
Charizard's attacks (with some exceptions) come out quickly but have a hefty cooldown, leaving him open to punishment should his opponent properly bait his attacks. His large size also makes him vulnerable to projectile spam and pressure, though he has many tools to clash and beat projectiles that can be attacked, such as his nair, jab, ftilt, and glide attack. He also has many effective out-of-shield options for escaping pressure, such as his upsmash, up b, and low-hitting aerials.
That's my take on the character, at least. Good luck working out the rest!