The way we choose to decorate our homes is incredibly personal. Much like the way we choose to clothe our bodies, the way we keep and dress our homes often says something about who we are, what our interests are, what our lifestyle is like, what our beliefs and values are, etc. The physical environment we create in our home can be designed to express our personalities, attributes and characteristics, as well as to create an emotional atmosphere which can have an impact on us and our visitors.
When preparing to design a space, you should begin to investigate what kinds of styles and themes you prefer. One suggestion of how to do this is to observe other spaces and objects (ex: pictures in magazines, images online, TV shows, show-homes, homes of friends and family, go window shopping, etc). When you find something that appeals to you, dissect the space/object to discover exactly what it is that you like about it (ie: design elements and principles). Ask yourself the following questions: What kinds of lines are found in this space? What shapes? What colours? What textures? What kinds of patterns do you see? What design principles do you see at work? etc, etc. Chances are that if you are finding a lot of curvy lines, and ornate details in the spaces you seem to like - you will probably like things in a classical style. If straight lines and minimalistic images appeal to you, you will probably like spaces that are quite contemporary. Once you know exactly what it is you like, it will be easier to create components and find objects to put together in your space.
You may also want to ask yourself about the emotional atmosphere you want to create in your space. Ask yourself questions such as: What purpose do I need this space to serve? What ideas/messages do I want to incorporate into this space? What feelings/impressions do I want to create upon people in this space? Again, visit, or observe images of other spaces (homes, businesses, public spaces, etc) and observe what your feelings or impressions are in that space. Once again, dissect the space and try to determine just what it is that contributes to those desired feelings or impressions (ie: colours, lines, patterns, lighting, art work, photographs, etc). Analyze your thoughts and emotional responses to the particular details you find. Doing this will help you to know what objects, design elements and principles to include in your space in order to help you achieve your desired effect.
Now that you know what you like and what you need, you're filled with inspiration and you're ready to begin designing your space!