kacangnim:
It depends on the program. Not every program requires work/internship experience. If you are applying from undergrad, it's a given you won't have full-time work experience - even a year. An internship is a great way to show you're gaining experience and, more importantly, pushing yourself. At the same time, volunteer work, extracurricular activities, and other forms of college/community leadership go a long way.
I would attend information sessions for the various MS programs you are considering and ask how internship/work experience is viewed. Do students in the typical class at these programs come with experience? If so, how many or what percentage?
Remember, there is no one-size-fits-all here!