Soldier
If your ancestor served in the British Army or the Honorable East India Company Army, (HEIC Army) then you may find his military records very useful in providing details of his life and history. More details are given on the pages just mentioned. Until 1859, the two armies were entirely seperate and their records were stored seperately. The HEIC Army was abolished in 1859, and its regiments were subsumed into the British Army.
See List of HM Regiments for British Army regiments that served in India and some information about them.
The FIBIS search page has an alphabetical list of British Army pensioners who elected to reside in India between 1800 and 1857.
If your ancestor was in the British Army, and you only know that he was in a certain location at a certain time (e.g. he had a daughter baptised in Meerut in 1859), then you may find John Kitzmiller's book In Search of the Forlorn Hope : A Comprehensive Guide to Locating British Regiments and their Records useful. It lists all British Army units by location. This website lists the India related material from the book.
Once you have determined in which regiment he served and at what times, you can learn a great deal about the experiences of your British Army ancestor by obtaining the regimental history of his unit. The National Army Museum may be of help in this respect.