The standard rules of Adventurers League character creation apply, with one exception. You are permitted to have a character with Fly at first level. You can read those rules here.
You may make your own character, use a pregenerated one, or may request character sheets, within reason.
Most of the adventurer listed on this website are story heavy, including a few adventures that do not contain combat at all. This is purposeful, as Monty's strength is in having a narratively interesting adventure.
All of the adventures are designed with Level 4 characters in mind, but if you'd like something stronger, just include it in an email. Attack at Feiler's Roost is the exception, designed for level 2 characters as an introduction into playing D&D.