You learn how many watts you need by googling the CPU followed by TDP example (AMD Athlon X4 880K TDP) and googling teh video card the same way ( GTX 1050 Ti 4GB TDP). Then add about 100W to get headroom. Then round up to the next larger PSU. I did not do the googling, but above posts are accurate.
The reason you do the googling rather than trusting the forum append is that it takes 2 mins to do, and then you KNOW you have the right answer. You do not have to worry the poster got it wrong, or got the wrong part #.
MORE IMPORTANT than wattage rating is quality of the PSU. You do not want 600W of dirty power with ripple and voltage problems. You want nice clean power with working protection circuits. Use this list and get a high tier PSU: http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html