Trying to organize a den in your home needs to start with what purpose it will serve. There are various purposes people use a den for. It could be a masculine "lion's den" that houses the large TV and is meant for sports nights for the lord and master of the house and his friends. It would include other gadgets and facilitate beer-drinking weekends. On the other extreme, it is a large living space where the family gathers for intimate evenings. A library, a home office, etc. is other forms the den can be created for. It is assumed here that the den is being organized for the first time, rather than being a remodeling project.
Having decided what the purpose of the den is, your organizing decisions will largely depend on the size and shape of the room. A regularly shaped, a square or a rectangular room, is easier to set up. Other shapes pose some difficulties in deciding what goes where. A family room will have the main TV fronted by a comfortable and large couch. That helps the family to sit together and enjoy quality family time. This also necessarily means the windows in the room need to have heavy drapes to darken the room adequately when required. A direct consequence on the room lighting is that you need a dimmer for reducing the internal lights to comfortable levels when viewing the TV.
In contrast, when the den also must accommodate a home office, you would want a lot of natural light to come in and place your work table and the visitors' chairs in a corner that is well lighted. If the window is large and floor to ceiling kind, nothing like it. What books you want to display in either cases, could go into the floor to ceiling bookcases. Other furniture that goes into a den in case of a family room is a writing desk with a table lamp and a chair. In case of the library avatar of the den, you will have more of the bookcases, a couple of comfortable reading chairs and a desk with a straight-backed chair, in case you need to take notes, add notes to the margins of the book and so on.
In any of the forms of a den, wall color should be some warm and light color of your choice. A light color provides an impression of open, airy space that this kind of intimate space deserves. Window screens need to have light screens beside the heavy drapes discussed already. When not viewing TV, the light screens will allow plenty of natural light to come in. A den for kids will have some additional considerations, of course.