Many high school students begin their university admissions too late - they concentrate on boosting their marks in their senior or junior year. Getting admitted to a chosen college or university requires a little of forethought and planning; at the least, getting admitted to a particular college does.
At first, you should work on your marks. Your mark point average does not tell colleges you are smart. They tell them you are willing to work for a long term and abstract goal. About 40% of university entrants graduate, and the greatest differentiator is not intelligence, it is the desire to work hard. That is what an apt GPA shows.
You should not pad your GPA by choosing dummy courses; universities are aware of such a trick. In high school, you should balance taking challenging with getting good marks. As to how high your marks must be, higher is better, still a 3.25 or higher is sufficient.
When you’are balancing your should keep your GPA with taking university preparatory courses, do not forget to have a personal life. Universities weigh many candidates. They seek for candidates who not just cram their way through university, but have a part-time job, also do some community service, as well as are active in churches, play sports and have other extra activities. These inform a university that you are going to become a full and well-rounded person, and realize the necessity to be active in the community. A great set of community service and optional activities can outweigh deficiencies in the mark point average.