Geography uses the concepts of place and space to explore environmental patterns and processes.
This is a very important section and if you look at recent Preliminary Examination papers you will find that many questions were flowing from this section.
You have to be through with almost all topics covered under this section as question from wide range of topics from Indian and World Geography is being asked.
Be clear about the climate, agriculture, minerals resources and energy, national parks and wildlife centuries, industries and transport as you may find many questions from these sections.
What to do?
NCERT books are quite helpful in gaining familiarity with Geography. It is quite interesting subject and Map section is the most important area to cover. A good Atlas is a must for any Civil Services Examination aspirant.
Books like India a Comprehensive Geography (D R Khullar) and Human and Economic Geography (Goh Chang Long) are quite useful in gaining knowledge.