Candidates those who have passed the Class 12 board examination and are appearing in the same, both are eligible to apply. As you are studying in Class 12, preparing for AIIMS, and your boards are to take place in the month of March, so in the Qualifying Exam Category you need to write Class 12 board examination and within the sub category you need to fill up as appearing/result awaited. As you fill up the sub category the column for marks in each of the subjects will appear as NA, and you can continue filling up of the rest of the form.
I hope this helps.