Hi Tanu,
You can take the help of sobject history in your apex and built a query matching the sobject id with the related id field in history.
Exapmle : If you want to track field history of Account object, you can use
SELECT AccountId,CreatedById,CreatedDate,Field,Id,IsDeleted,NewValue,OldValue FROM AccountHistory
Similarly If you want to query field history of custom objects, you have to match the parent id of history query with the custom object id.
Example: If you want to track field history of Phase custom object, you can use
SELECT CreatedById,CreatedDate,Field,Id,IsDeleted,NewValue,OldValue,ParentId FROM Phase__History
Hope this helps you.