Company audit is a vast subject. It requires Act knowledge and practical experience. However, to start with first you need to learn the following:
Basic Principles of Accounting, Accounting Standards, Auditing and its principles, Business Law, Commercial Law, Company Law, Indian Taxation like Central Excise, Service Tax, Income Tax, Vat etc.,
if you list it will go on........ a big list
I feel you better Join A practicing CA firm and get trained. Practical experience along with the theory study gives step by step knowledge. Purely book studies is not sufficient.