Soldier
If your ancestor served in the British Army or the HEIC Army, then you may find his military records very useful in providing details of his life and history. 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.
Military Records on Microfilm gives a brief description of the main types of records available, with an emphasis on those that can be obtained using LDS microfilms.
H.M.’s Regiments has regimental histories of many British Army regiments.
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.
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. Much of the material found in the Regimental Histories section of this website has come from the National Army Museum.