Technically, you can't crit an ability check. Otherwise you could make an unskilled acrobatics check at level 1 to survive a two hundred story fall and succeed because you rolled a 20. If a 19 is instant death and a 20 is a perfect success, it's kinda wonky. Obviously it's allowed if you aren't playing seriously, though.
As a lot of other people have mentioned, your best bet is to start playing. That's the best way to learn the mechanics. Except maybe playing a videogame like Neverwinter Nights. But honestly, DnD isn't a mechanics based game. You can play the entire thing without rolling a die. What you need is a role to play, and people who will help you play that role(DM and players).
JoCat's vids are mostly memes, but you should have an idea of the class you want from that, right? If not, you can narrow it down to what you want your character to do. Magic, skill, swordsman, so on. The more details you work out, the easier it will be to pick your class. In most cases, what you want will be in the game already, but you can also just make something up as well.
Once you know your class, you can make your character. Hmm? The order's off? Whatever. You can make a backstory, and that backstory will affect the mechanics of the character. Nobleborn? You can just walk into the mansion that everyone else has to sneak into. Musician? The innkeeper will give you a room free of charge if you entertain his guests, and people from towns you've performed at might recognize you.
It's essentially the same as writing, only you spend all your effort on a single character. If you know the DM, the two of you can collaborate to make a story that fits in the DM's world. If not, they'll have to scramble to make it work, but that can also be fun. Just don't make a Mary Sue if you want the DM to like you.
If you have a character in mind, explain it and we can help you define it mechanically. Also, if you don't care about the role playing and just want to make the strongest character ever- well it's not impossible. There's a lot of min-maxy DnD content on YouTube.