Most American universities require Common Application: https://www.commonapp.org/Login which can be sent from August. In this application, you provide your data and grades and choose universities you want to apply to.
You will also need letters of recommendation from your teachers. You have to write an essay on a chosen topic, maximum of 650 words long. Apart from the Common Application, around 2/3 of universities require additional applications, usually essays – details are available on each university’s websites.