Vartika, hope you are doing good. At this point in time, NEET 2021 registrations have not begun yet. And they might be released tentatively in the month of January because we are already past 15 days in December. Nevertheless, there are 5 stages in the Application process:
Registration
Filling the Application form
Review and Final Submission
Upload Photograph and Signature
Pay the fees
Take a print out of the Confirmation mail

You will need the following documents to be uploaded while filling the application form:

Uploading of Photograph: Instructions

Must be latest passport picture in a white background with the Name of the candidate and date written on the photograph. The Size must be 50 Kb to 300 Kb and format should be JPG.

The focus has to be on the face with 80% coverage and ears clearly visible.

Uploading of Post Card size photograph: Instructions:

Must be latest post card size picture in a white background with the Name of the candidate and date written on the photograph. The Size must be 10 Kb to 100 Kb and format should be JPG

Uploading of Signature: Instructions

Candidates must sign with a black pen on a white background in JPG format with a file size of 4 Kb to 30 Kb

Uploading of Left hand thumb impression: Instructions:

The file size should be 10kb-50kb in JPG/JPEG format.

Class 10th Passing certificate: Instructions:

File size of 100kb to 400 kb in JPG/JPEG format.

Most Candidates wont have their 12th certificates by then and so it is not mandatory yet to upload 12th certificate. During Counselling registration process, you need to upload both the marks sheets. For NEET application, it is fine even if you do not upload your 12th marks sheet.