Decide what career path you want to take. This may seem simple, but graduate education is far more focused on a particular area of study, so making sure your potential major fits with your desired career path is crucial.
Figure out what degrees are necessary for your chosen career path. While some careers, such as many in the health field, require you to have a master's degree, job experience is more important for others. Researching the expectations of someone in that field is important and can be done by finding companies looking to hire someone of your chosen career and seeing what they require.
Decide if you want to focus on the same field of study as your bachelor's degree. Many people decide to go to grad school to study fields not directly associated with their undergraduate studies but that will help with their careers. Sometimes having a well-rounded education can benefit you more. If you do decide to stick to the same field of study, pick a specific part of that field that you enjoy and would benefit the most from.
Choose a program that you have some experience in. You don't need to have focused on your potential grad school major, but you will need some experience and competence in the field in order to get into the program. If you have a degree in math but want to focus on something in science during grad school, make sure you're ready for the science courses.