1. First identify the application and what is to be tested.
2. Identify if really tools are required (say automation). 
For early stage testing, automation is not recommended. 
Application should be stable for automation.
3. Carry out the product features study for different tools 
available in the market and check if that suits your 
requirement.
4. Check the tool usage budget allocated to the project
5. Check the value add and time reduction by the tool usage
6. Check the expertise available for the tool usage
7. Finalise the tool based on the points 1 to 7 and go for 
it.