You could start with a particular example. To make the point more clear, try beginning with a drastic example. Suppose some person (let's call him Kasper) is born and his parents keep him in a house with no windows, no Internet and no access to the outside world. To Kasper, the world is that house. When he is young, he doesn't even know that the outside world can be accessed. His parents surreptitiously retrieve supplies when they need them, always while Kasper is asleep. So, for the first part of his life, the universe is only as large as the inner rooms of that house. Suppose that his parents have been educating him on the basics (reading, math, science) but that education is supplemented by his parents strident racism. When they think he is old enough, they tell Kasper that there is a world outside the walls of the house and that his job is to protect "people like him" and to hate others who are not like him.
Ideally, Kasper will develop enough intellect to logically question his parents' teachings and hopefully he is able to overcome their racist brainwashing. But from this example, consider how a person's environment has a large role to play in their understanding of the world and how this affects their free will. If Kasper had never gotten out of that house, he might never have had the opportunity to learn of other ideologies and theories of life. It is often the case that children learn the most from their parents. Parents who allow their children to learn as much as they can do their children a great service. The more a child knows, the greater the capacity for free thought. For example, if a child learns there are only three ways to live life, that child has three choices. If a child learns that life is what you make it and/or there are innumerable theories to follow and ways to live, that child will have many more choices. More knowledge = more choices = more freedom.
You could also start with a more particular kind of freedom. Democratic citizens generally enjoy more political freedom than citizens suffering under a ruthless dictator. Again, freedom is dependent upon the individual's environment (parents, culture, economics, political system, even genetics). But with the right outlook (and this comes back to knowledge and the courage to think outside the box), an individual can choose to at least think more freely than his/her environment will allow publicly. So, at least in these terms, freedom can be expressed mentally and/or socially. For example, if I live in a repressive society but I educate myself about other ways of living, I can at first think more freely than my society dictates and maybe I can even recruit others and change that society. We've seen such uprisings in the recent and ongoing revolutions in the Middle East.
Freedom requires the possibility of choices. And to be truly free, we must be willing to accept responsibility to make those choices. If I do not accept responsibility for my choices, then those choices are not really mine. Therefore, it also takes courage to be free because one must be brave to accept responsibility for all his/her choices.
There are a lot of things to think about with the issue of freedom. Knowledge, choices, and responsibility are among some of the significant issues. In the end, freedom is also about overcoming environmental factors (political, genetic, cultural, parental, economic, etc.) which limit freedom by determining an individual's choices.