There are programs on the Internet that allow you to input all your computer details and they tell you the WATTAGE you need. Some programs are more up to date and/or better than others.
To be clear, you should know:
1) the approximate Wattage that you need
2) the Amps you need for your graphics card (get at least 25% better than your graphics card rating. For example if the card needs 10A, then 10x1.25 is 12.5Amps minimum on the single or combined +12V rail/rails.
3) connection types, such as 6-pin or 8-pin which are on the graphics card but which might not be on the PSU.