First go to your Profile page and click edit, then in the Raid Planner section click Add new character
In the popup box that appears click on Add new Character
Fill out the details of the Character you wish to create, then click on Save
Then from the dropdown list choose your new character
The new character should appear in your character list, when you are finished click Submit
If you navigate to the Raid Roster for the guild you assigned the character to, the character you just created should appear
Now navigate to the Raid Calendar, and choose an upcoming raid you wish to join
Clicking on the raid will bring up a popup window with info about the raid, if you want to signup click Signup
First choose your character from the dropdown list, then choose your attendance status and the role you will play in the raid; when you are finished click Save
Your character should now appear in the raid signup page