I'll give my input as a casual tanking PvP player. Hence BG are always the atrocity called random battle grounds.
I'm assuming that you're joining in some kind of tank spec, possibly in your main tank spec.
I'm also assuming that you have access to a semi-decent set of PvP gear. Say the 371 you collect for honor. If you don't, just don't bother. You're made of paper the moment a caster shows up.
You'll be doing bloody awful dps, so your goal is to stay alive in a constructive way. Or you'll suicide in an equally constructive way.
When suiciding, try getting a message away that you're planning to do so, and that you don't want any help. Your mission is to buy the rest of the raid a few extra seconds by keeping some opponents busy killing you. Just burn every CD you have available for survival and pop whatever CD you have for getting out of CC.
When staying alive you're either waiting for reinforcements to arrive, in which case you do as when suiciding, or you're jogging around with a flag. In the latter case you're trying to reach teammates unless you already have an escort with you. Use defensive cooldowns sparingly if you have a healer with you. In that case, attack as many targets as possible. You want people hitting you and leaving your healer alone.
When people refuse to hit you, or you're getting chain CC:d, Hand of Sacrifice a friendly target who's taking a beating.
I'll follow up with some more specific articles for casual BG-playing as a tank.