Selecting subjects can be a bit intimidating at first. But, there are lots of resources available to you, including this book, to help you select appropriate subjects. Also keep in mind that there just aren't all that many general subjects from which you will choose, especially before a child reaches the junior high level (typically considered seventh grade).
As you contemplate the subjects you are going to teach, refer back to the legal requirements for your state. If your state requires that specific subjects be taught, this will provide a good starting point for your selection of subjects for a specific year. Some states require specific subjects for specified grade levels, while others require that a general set of subjects be taught each year. You can refer to the HSLDA Web site for your state to determine if subject requirements are provided.
For example, consider New York's subject requirements. New York requires that specific subjects be taught and also provides the grades in which those subjects must be taught. All grades (K-12) must be taught the following subjects: patriotism and citizenship, substance abuse, traffic safety, and fire safety. In grades 1-6, the additional subjects required are arithmetic, reading, spelling, writing, English, geography, U.S. history, science, health, music, visual arts, and physical education. Although this is quite a list of subjects, the 1-6 grade subjects don't all have to be taught in the same year, although some of them (such as math) are likely to be taught each year at a different level.
Utah is another state that requires specific subjects be taught, although the list of subjects is not broken out by grade level, as is New York's list.
Although at first such subject requirements might seem burdensome to you, they can actually be helpful especially when you are just starting to homeschool because you don't have to come up with a list of subjects from scratch. And, although these subjects are required, you are still free to choose how you teach the required subjects. The requirement is only for a general subject; your job will be to decide how you want to teach that subject and the kind of curriculum you will use to teach it.
You should consider such lists of subjects to be just what they are basic requirements. You are free to add more subjects, and you probably will want to do so.
If your state doesn't require that you teach particular subjects, you are free to choose the topics that you believe should be taught. This can be more difficult initially, but choosing the general subjects for a school year likely won't be all that difficult for you, especially after the first year.