If you are considering choosing computer science, your post-secondary planning process should begin as early as high school. Computer science programs can be competitive, so it's important to plan your high school courses accordingly. Computer science students tend to struggle for reasons that have very little to do with computers. More often than not, such struggles are rooted in weaknesses related to math. So load up on math classes. Focus on algebra and if you have an opportunity to take another math, go with geometry. Trigonometry will be useful too, but you will gain a lot of this knowledge in high school math classes even if you don't take a specific class in it.
If you are still in high school and are thinking about taking computer science, there are several additional things that you can do to prepare and increase your chances of being accepted:
Undertake to read and study independently. This is a good way to develop interests, expand your knowledge and improve your vocabulary and reading comprehension skills needed for school. Computer science students often have the greatest difficulty with writing and English classes in their first year of post-secondary study.
Gain experience in the computer science field. Job shadowing, talking with a professional, or attending a summer or spring break computer science camp are all recommended activities.
Admissions committees may also be interested in a student's personal qualities, which can be developed through participating in extracurricular activities. Consider joining or even starting school clubs, whether related to your area of interest or not. This is also a good way to develop demonstrated leadership skills, which are an important aspect of any competitive application.
Work experience - paid or volunteer - can also increase your chances of being admitted, particularly to programs involving internships, and is an achievement often regarded highly by admissions committees.