An ideal is a defining Idea or moral stance that overwhelms other beliefs you might have.
How to Choose a good Ideal.Edit
Think of your personal morals or opinions, then consider how those might impact how you will play the game. Use this information to form the Ideals of your character. Now, if you are looking for a challenge, pick the opposite and each time you go against it take a -1 charisma.... just to be crazy.
Because a character made up of numbers is boring, and this will tell the DM your intentions.