Traditional textbooks have that already figured out for you. If you choose a different route, you need to look at a standard scope and sequence. These are available from books, your state department of education, or as a last resort your local school district. Once again, decide on a method but understand that it's perfectly okay to change methods when you try one that isn't working. All of us have partially-used curricula on our shelves that just didn't fit us.
Cost is another factor to consider when choosing curricula.
That is, of course, a fraction of what states spend on public school students and a fraction of the cost of private-school tuition. But many homeschoolers have done it successfully on less. When planning your budget, include outside lessons such as piano or dance. Ask your state homeschool organization about used-curriculum fairs and don't forget to utilize your local library. Remember, you can sometimes reuse a curriculum with younger siblings or sell it when you're finished.