1. Create a new character
To create a new character, click on the "Characters" link in the blue nav bar going across the top of the screen.
Once you're in the list of characters, click on the "New" icon (seen below).
2. Add your character's info
At the very least, your character needs a name, but that's about it. However, since they are a PC and not just some disposable NPC, it's worthwhile to give some backstory in the bio section. It will give adventure log readers a better idea of who your character really is.
To make this character into a PC, simply select the campaign to add them to in the drop down box, like in the image below. Only campaigns where you are a player will show up in the box.
Note: It's highly recommended that you make your PC public, as otherwise no one else will be able to view them, not even your GM or other members of your group.
As soon as you save your changes, the character will be added to the campaign!