Do I also need to register for aiims before its result ?

No, you don't have to register now you have to attend the counselling for allocation of seats.

To check the process of counselling you can refer  below link

AIIMS MBBS Counselling

How much marks are you expecting in AIIMS?

The counselling of AIIMS MBBS 2017 will be conducted in three rounds beginning from July 3 to 6, 2017. AIIMS MBBS 2017 Counselling will be conducted for admissions to as many as 707 MBBS seats located in 7 AIIMS institutions across the country. AIIMS MBBS 2017 counselling will be conducted in offline mode at Jawarharlal Auditorium, New Delhi. Candidates are requested to visit in person to attend the counselling as per their schedule allotted against their rank.

To know more about the same, you can go through the below article -

AIIMS MBBS Counselling 2017

Hope this helps!!!