As you have asked about the AIIMS online application process for MBBS, I want to tell you that
AIIMS MBBS 2016 Registration Process will be going to start on last week of January (Tentatively).

So you have to wait till then to apply for the AIIMS.
You can register for AIIMS MBBS course from the AIIMS website, when it will be available.

Some eligibility details:
Age:-
Candidate should have attained or will attain the age of 17 (seventeen) years as on the 31st of December of the year of admission
Qualification:-

Candidate should have passed the 12th Class under the 10+2 Scheme/ Senior School Certificate Examination (CBSE) or any other with ENGLISH, PHYSICS, CHEMISTRY and BIOLOGY as subject.

Contenders who have appeared or are appearing at the 12th class Examination with English, Physics, Chemistry and Biology as main subject and expect to pass the Examination with required percentage of marks can also apply
The minimum aggregate of the marks in ENGLISH, PHYSICS, CHEMISTRY and BIOLOGY obtained in the qualifying examination are:

General and OBC candidates: 60 %
SC/ST/OPH Candidates: 50 %

Exam Pattern:
Subject - Number of Questions

Physics - 60 questions
Chemistry - 60 questions
Biology (Botany and Zoology) - 60 questions
General Knowledge - 20 questions

Total duration: 3.5 hours