I'm also doing As Level German but something which helped me bridge the gap between GCSE and A Level was either through writing in German on on-line forums or just through thinking in German, translating as people speak to me to German or just using a medium such as Skype to talk to Germans who will correct you whenever you make a mistake, and I mean whenever!
But also because I did this through GCSE it helped that I had already started learning the cases and adjectival declinations, pronoun changes etc.
The key part is knowing enough vocabulary and grammar, as well as getting over the initial anxiety of speaking a different language and just speak! A key skill that you will need to develop is described well in an analogy of my teacher. It's called the "Word Wall", as soon as you come to a word that you don't know how to say, you need to climb over the word using other words to mean the same thing, like doing sports, it is hard at first but as you practice it, you become quicker at it and you become more confident in doing it until it's just a natural thing for you!
Hope this helps!