Getting into graduate school can be more selective than getting into college so your grades are extremely important. The higher your GPA the better chance you have of getting accepted. Depending upon the school, a 3.0 GPA might not be enough so you should be aiming a lot higher. Also, you will need at least two recommendation letters from individuals who can speak to your character, work ethic and leadership skills.
Just as important, you will need to do well on the standardized test required for the type of school you are considering. Business schools require the GMAT, law schools the LSAT and medical schools is the MCAT. Again, the higher the score the better your odds of being accepted. Note that some schools are starting to waive the standardized test requirement so you'll have to look that up for each school when it gets closer to applying.
Finally, many schools may want to see some real world experience on your college application. This can take the form of internships or participation in clubs related to your major. As it relates to clubs, if you held some sort of leadership position or at least lead a community service project, that would be ideal.