Google ESO AvA or PvP tips/pro tips and you'll get lots of videos to watch (if you're into that sort of thing). Here are some reading resources, mostly geared to starting out in AvA. You will find build tips, siege tips, and small and large group tips.
http://forums.elderscrollsonline.com/en/discussion/232232/pvp-for-beginners short read on skills and counters
http://forums.elderscrollsonline.com/en/discussion/172558/surviving-cyrodiil-how-to-hopefully-die-less-often/p1 longer and more inclusive tutorial
http://deltiasgaming.com/2015/01/07/eso-pvp-builds/ some sample build for each class
http://esoacademy.com/eso-pvp-and-cyrodiil-guide/ hard numbers on siege damage
I did watch a few videos, some were good, others not so much. Couple tips I picked up, shoot for 30% of group as dedicated healers...and protect them, stay "loosely" bunched together around "Crown"...if you run off or get separated from your group you're easy picken's, tanky'er builds draw aggro and keep enemy CC'd, DPS kill (while protecting healers), healers stay out of melee scrum (back from fight), and rezzing during battle will often turn the tide for you, don't just stand on the bodies of your fallen, get 'em up.
I invite any of our more experienced PvP'ers to chime in here, and validate the info above or give more current info, or some tips on what we are missing.