Usually, the first few pages in an online college application concern personal/demographic information, such as birth date, home address, telephone number, race, etc. You may also be asked to form a short list of any extracurricular activities and hobbies in this section.
If you are just beginning your undergraduate career, you will have to provide high school information and final GPA. If you are applying to graduate schools, you will need to provide your current college's or university's information and final GPA. In both cases, you will also need to send proof of your grades (report cards or transcripts).
For undergraduate applications, you will have to fill in SAT or ACT information, as well as send separate proof of your SAT or ACT scores. For graduate applications, you will have to fill in GRE information and send separate proof of your GRE scores. Some graduate schools may also require GMAT or GRE subject test scores.
Find a teacher or a professor that you trust to write a letter of recommendation for you. You may have to send these in separately. In some cases, you can provide your recommender's email address, which will allow the person giving you a recommendation to upload the letter directly to your application.
The length of the statement of purpose may vary from application to application. In almost all cases, however, this is an essay in which you should describe your personal background, educational background, personal interests and explain why, specifically, you are choosing to apply to this particular college. In most online applications, there is a text box that will allow you to copy and paste your statement into the application.
Many universities may require you to upload a file of exemplary work--often examples of research or an essay. They may also require a resume listing your work experience.