Help in structuring and writing a literature review

I am beginning my dissertation and for the first part we've been asked to do a 3000 word literature review in order to get some kind of background into our topic and get to know it. However, I'm feeling quite overwhelmed and I don't really know where to start. I was wondering if someone could give advice on how they structure a literature review and the best way to approach it.

I don't want to say my exact topic but it's on the effect of hormone 'X' on anxiety. Anxiety is so complex with so many different theories and I'm just finding it difficult in knowing how to even start this paper.

Also I'm a third year undergraduate and we've been told to use mainly primary sources (i.e. actual studies) rather than review articles. Argh just feeling overwhelmed. I have one month exactly to do this.