simple

1. how much power does your CPU, GPU, HDD, etc etc needs?
2. check each power requirements on the following items to get an idea on how much power is required
3. for eg. i7 4790 eats a maximum 84W, GTX760 eats max 175-180W, average consumer HDD about 5-10W or so
4. add them up and youll have an idea of how much power it theoretically consumes (bear in mind most of them never actually reaches the peak TDP constantly)
5. now to PSU part, how to pick the right wattage
6. check the brand for reliability, then check the labels on the PSU. see whether does it provide a stable supply at load. do own R&D on how to read and understand PSU labels
7. then pick the correct wattage based on what you have already learned and understood
or in short, most manufacturers already put recommendations on how much wattage PSU is required for that particular PC hardware. say like a GTX760, Nvidia already said it requires at least a 500W and above PSU with decent ratings

Hi.    well if u have time, u can also use the http://pcpartpicker.com/

Just simply pick the parts of your pc that u'll be going to use.

In there, the wattage of items u pick will automatically calculated, hence can give u estimated wattage needed.

Here's example of it.

http://pcpartpicker.com/parts/partlist/





Regards.