Dear,
You don't have to worried.Although the competition seems high but there is nothing a little hard work cannot help you achieve.You should start preparing for the medical entrance exam by first making your Physics ,Chemistry and Biology strong.
1. You can refer different books like Cengage,Dinesh,Pradeep,Irodov books to have your basics clear and get a better knowledge abot the topics.
2. Different coaching institues are present all over India .They conduct online test series to help you practice for the exam.You can appear those to help you get aquainted with the test and question pattern of the examination.
3.In AIIMS , along with Physics ,Chemistry and Biology the give questions related to General knowledge and Reasoning .Try keeping an update about the happeing in the world.Read newspaper every morning.There are general knowledge books or online you can get current affair question. Practice those.Many reasoning books are available in the market.Practice from those too.
4.Make a timetable.Divide your time equally and give each subject importance. Work a little harder.
I wish you luck.  
If you have any further queries you can ask it we will surely answer it as soon as possible.