Friend for the preparation of mains exam you can choose any subject in the list optional subject authorized by UPSC for mains exam.
List of optional subject for mains exam
Animal husbandry & vetinary science
Political science & international relations
In the above list you can choose any subject of your desire.
As you mentioned that you have completed graduation from botany subject, so I think you would have good knowledge of botany.
So you can choose one optional subject to botany in preparation.
And as other subject you should chose History, Geography or Psychology.
These are easy subject will also help you in other paper like general studies.
History and Geography both will be helpful in the preparation of GS.
There are two papers of general studies in prelims exam and also in mains exam.
But friend finally you should choose subject as per your interest and caliber.