As you have asked about the online application process to get admission in MBBS in AIIMS, it has not opened the portal for online application as on 24th Dec, 2015.
However you can check it regularly from its website.
To apply online for the AIIMS, go to AIIMS website,


Then click on Students, the page open look like



Then click on MBBS, the page open look like





Here you will find the link for the online application for MBBS for AIIMS.

Important Dates:
M.B.B.S. 2016
Entrance Examination 29th May, 2016 (Sunday)
Result 14th June, 2016 (Tuesday)
1st Counselling Dates will be declared later.
2nd Counselling
3rd Counselling
Open Counselling (If required)

Contact information:
Examination Section, All India Institute of Medical Sciences,
Ansari Nagar, New Delhi 1100608
Our Location : Convergence Block,
1st Floor [ Mon-Fri 9.30AM -5.00PM, Sat 9.30AM-1.00PM]
Telephone Numbers : 91-11-26588500, 26588700, 26589900
Fax Nos : 91-11-26588663, 26588641