Hello Simmi, 

AIIMS consists of a total of 200 Qs.

60 each from bio, chem and phy. Apart from this it has 20 Questions from GK and logical reasoning.

The questions asked are mostly of basic level.

They are divided into current affairs n GK, logical reasoning and aptitude.

For current affairs you need to stay in touch with a news daily lik India today,The hindu, Times of india etc. Pay special focus on the major headlines or events. 

You can refer certain books which are provided for UPSC, CLAT, SSC preparation. 

Go through certain portions of general knowledge like inventions, diseases, general science, geographical facts, important dates, mythology etc. 

Coming to the logical reasoning and aptitude portion, the questions are to test your general logical ability, so they are basic questions and wont cause that much trouble. 

All the best :)