If you are applying for a Tourist visa, you actually have 2 options. One is to apply online which I did and the other is to go to the Visa Application Center and submit your paper application. This post is about online application. You would have to determine your eligibility first.
After you determine your eligibility online, it would give you a unique number and ask you to sign up. I signed up with GCKey.
Once you have signed up, I applied for a tourist visa and it listed the requirements I need to fill up and documents I needed to upload.