It is important to determine which GED areas you are weak in. Many study books will have pre assessment tests to help you determine what area/s of the book you should start with. This is especially important when you are studying math. Consider each piece of math as a building block. You need a solid foundation of basic math (whole numbers, fractions, decimals, and percents) before you can successfully handle algebra, measurement and geometry.
If you have access to a learning center where a GED Instructor can assess your skills, have them help you determine where to start.
It has been my experience that the better your reading, the faster you will proceed through the GED.