How to get Isabelle
Although you are in charge of our own island as mayor, you have no direct help from the start outside of Tom Nook. Those who have played previous games should know that Isabelle is your secretary in the games, but it looks like she was a bit busy with Super Smash Bros. showing up from the start of the game.
Isabelle is a character that you won't even see for a while either, as you have to complete a number of tasks which will take several days before you get to your island. Basically what you need to do is start the game and complete all the tasks given to you by Tom Nook and company.
This includes getting the Blathers to come to your island and open the museum, opening Nook's Cranny for Timmy and Tommy Nook, building a bridge, and most importantly, getting three new residents to move to your island.
It is the latter that will delay the process the most, as you have to scramble to recruit new villagers for your island. This can be done by flying to other islands at random using Nook Miles tickets, so make sure you go as much as possible and recruiting three new people to your island.
Tom Nook will follow you around to find plots they can build on and get all six required items for each house before they open. In addition, it will take a few days for all of these residents to move into their new homes.
After completing all of the above tasks, you need to wait until the next day. You will notice that the Resident Services Tent is now closed for the day, but it will reopen the next day as a remodeled Resident Services Building, with everyone's favorite Shih Tzu Isabelle located inside.
