Hi i jyst bought a Berkley baitcaster combo and i was looking at the specs and it shows that the reel can handle 12lb test. I want to put braid on it. Can i put 30lb test braid on the reel. Thanks.

Certainly, 30lb is the minimum I use on baitcasters because any smaller and it tends to dig in itself for me. And that cause backlashes and breakoffs. 30lb braid should also cast like a dream 

Listing on reel is only a guide line telling you how much line it holds of a certain size.  40# braid is close to the diameter of 12# mono.  So if your reel holds 125yds of 12# then it will hold approximately 125yds of 40# braid.  You can use 20# mono on the reel if you want.  It will just hold less than 125yds of line.  Won't hurt the reel.