Buck converter PCB traces width
8 Amp is a whole lot of current and you need to understand where all this current will flow on the PCB. After this is understood you can go ahead and design the trace width for all the different connections. Also care needs to be taken so that the pads on the PCB is sufficiently large to transfer all this current. This is very important, you do not want the PCB to get too hot. I do not see any fuse on your schematic but that could also be good to add.


  do I need to make the thickness of the traces same for the Capacitors


The current peak for the charge/discharge curve by a capacitor will be determined by the capacitors resistance and the voltage supplied. A capacitor of "normal" capacity will not be able to charge or discharge all that current (and this is perhaps not needed nor wanted), thus you will probably not need to make the traces as wide to these as you would otherwise.