Understanding Subsidiaries:

A subsidiary is a company that is completely or partly owned by another corporation that owns more than half of the subsidiary's stock.

A first-tier subsidiary means a subsidiary/daughter company of the ultimate parent company, while a second-tier subsidiary is a subsidiary of a first-tier subsidiary: a "granddaughter" of the main parent company. 

Ford motor company with its tiered subsidiaries is shown below.

Finding Subsidiaries:

1. Corporate Sites:

The best source to find subsidiaries of a company is its corporate sites itself. Press Releases provide information about recent acquisitions and other information. A lot of companies provide a lot of information about its subsidiaries in their annual Reports

2. SEC.gov

All companies, foreign and domestic, are required to file registration statements, periodic reports, and other forms electronically through EDGAR. Anyone can access and download this information for free. Here, subsidiaries list is available for most public companies.
Example: List of subsidiaries of General Electric at SEC.gov

3. Open Corporates

Opencorporates have information about 60+ million companies. You can search the database and find the parent company. There you can a huge list of subsidiaries for it. 
Example: List of subsidiaries of General Electric at Opencorporates.

4. Wikipedia

Wikipedia is also most valuable source to find acquisitions and assets of companies. There are a lot of articles covering acquisitions of various companies. In addition to that there a few lists which contain subsidiaries of a company.
Example: List of assets owned by Berkshire Hathaway, List of mergers and acquisitions by IBM