If you are apply to a 4 year college/university, they are looking for these three things:

Academics
- 4 year colleges and universities want to see an upward trend in your academics. Universities want to see that you are making your best effort in improving your grades every year. Maybe you didn't do so good freshman year, you did okay sophomore year, but you got your best grades junior year, that's exactly what colleges want to see. You can also go on each schools website and see what the average acceptance rates are for GPA's. But, don't think that you cant get into a school simply due to your GPA. Keep reading!
Activities list
- 4 year colleges and universities want to see that you are an involved member of your school, and/or community. They want to see what clubs you joined, community service you did, maybe you did sports. Start doing these things during your freshman year and by senior year you will have a well rounded activities list. The earlier you get involved the better!
Personal statement
- Your personal statement is all about you, so you should use it to showcase your personality, experience, achievements, and future ambitions. Universities want to get to know you, and why you'd be an asset to the school. Make sure that when you are writing your personal statement in senior year that you demonstrate your writing skills (punctuation and grammar), answer the prompts, and make sure you stay within the word count.

4 year colleges and universities are looking at ALL of these things. So make sure that you are hitting these three main components throughout your high school experience.

If you are applying to a 2 year/community college the process is much simpler. What they are looking for is:

That you completed your high school diploma or will complete your high school diploma
Then you apply. Community colleges accept any and all students at any time of the school year.