First, don't panic. The deadline is sometime next year.
When you first opened your account with them, didn't they send you a brochure explaining the steps needed to apply?
Don't worry about the number of envelopes. They will send you more later, more than you need.
Basically, you need to submit the following items
Use the provided envelopes:
1) Your High School transcript
2) Three recommendations from Math, Science, English teachers (ask your teachers to fill out the USMA recommendation forms)
3) CFA test results (ask you physical education teacher to do it only after you practice them enough)
4) Your ACT scores (send in your copy or ask ACT to send an official copy directly)
Submit online:
5) When you log in to your portal as a candidate, you will also need to complete the assays and submit them online.
Seek through your MOC:
6) At the same time, you also need to seek a nomination from either the congressman of you district, the 2 senators of your state, or the VP. To increase your chance, apply to all of them. Don't get stressed out. Their deadlines for a nomination are usually in Oct/Nov so you still have some time. However, there are some work involved. You don't have to know them, but you must first go to their website to download the forms for nomination. Complete the forms and send them in. Each one may have a different requirement. Some may want to interview you. That is normal.
Make Appointment for a Physical Exam As Directed:
7) You will also schedule a free medical exam. The results will be sent to USMA directly.
Those should cover most requirements. If I miss anything, others here can help add. You need to read that brochure or visit USMA's website go to admissions for the most reliable details. You also need to have a habit of login to your portal to view the updates.