Think through what you're going to do before you do it. Don't rush yourself, and don't let your group rush you. Recognise when you start to get really anxious and take a step back, steady yourself and get back to it. When you make a mistake, try not to get flustered and panicky, assess the situation, realise the options you have to stabilise the situation and enact them. Most importantly, when you're done, try to reflect upon what you did well and what you did not so well. You're just need to build some confidence. You aren't alone in your nervousness. I used to feel exactly the same, in dungeons, then in raids and arenas. It will pass as you gain experience and are better able to cope with situations that arise as you tank.