The question can't be discussed in such a short space. Members have already given some suitable advice. I am also going to mention some relevant points:-
1. Before filling up the form, proper planning is necessary for choosing the Optional paper both for the Preliminary and Mains. The optional paper must be chosen considering the candidates own likes and dislikes, not following advice/guidance of any other candidate(s) or teachers of coaching centres
2. The syllabus of the optional papers must be carefully noted and followed. For common optional papers (the subjects which are taught in schools), NCERT books on the subject from Class-VI to Class-XII are necessary to cover the entire syllabus. For other subjects, standard books must be followed. Only after covering the syllabus, special notes from the coaching centres should be studied.
3. Special mention is for General Studies paper. A candidate has to study a lot and study regularly for feeling confident in this paper. The questions can cover anything and everything. A candidate must study newspapers carefully and note down important issues in the diary. It consumes times. Reading of periodicals like Yojna, Foreign Policy News, CSR, GK Today, Manorama Year Book is a must. The candidates must have thorough knowledge about different schemes and projects of the Government of India. For this purpose, India Reference Annual published every year by the Publications Division of Ministry of I&B is suggested.
4. For the purpose of the English essay, in-depth knowledge of debatable issues is necessary. The candidates must be able to draw conclusions after discussion. For this regular practice is necessary. Analytical topics must be chosen and regular daily practice is advised.