I just finished listening through Adventure Zone Balance myself, and I can tell you they play fast and loose with the rules, with many mistakes, with the primary goal to have an entertaining podcast. It overlaps a lot with other ways to play D&D, like goofing off with friends, telling a compelling narrative story, strategic decision-making, but every table and game is different.
1 What to get? Little to nothing at first. A set of easily readable dice, if you're really sure you'll stick with it. Borrowing dice is fine if someone is willing to lend, or using a free dice rolling app if the group is okay. Save any future purchases until you've had a few games under your belt and you're sure it's something you and your group are going to stick with.
2 Rookie mistakes? If you're a spellcaster, knowing what your spell save DC is. It's easy to miss when creating a character, or knowing where to look on a premade sheet. If you ever do play a caster, such as a cleric or wizard, find this number, circle it in red ink several times, or better yet, write it in thick permanent marker on a notecard and staple it to the front of your character sheet. This number governs about a this of the things a caster does in combat, and losing or forgetting it is the #1 cause of slowdown I've seen in 5e.
3 Difference in dice? Most 'rpg dice' sets include the 7 you'll need. If you're at a game shop and they sell singles, as said above, another d20 and a few more d6 might be useful, to roll at the same time instead of rolling and adding. Don't accidentally get only a set of d6s or d10s (some rpgs use these exclusively) instead of the different set.
4 Keep in mind? This is designed to be a fun time with friends. Enjoy yourself, and look for ways to increase other people's enjoyment too.
5 Practice? Not... really. You can look up the stats of a goblin or kobold and have a one on one practice fight, but without someone overseeing it you won't be notified of any mistakes you're making.
6 Duplicate? Others have got this covered. I want to echo that many groups do a "session zero" where they create characters together, go over house rules, build out the setting and initial adventure, etc. It's okay to have a character concept in mind (and a fighter or rogue is good class to start out), be prepared to modify it to fit the group, or even abandon if you get a concept you like better. I might even suggest not even making your first character, and just using a pregen, until you get to know the group and the system better.