I'm taking that exam as well.
Imagine, the Spanish Civil War has never been on any IGCSE exam papers...it's the first time we get it and that means we can't go back on past papers or anything!
Basically, the paper will be based on Germany's involvement in the war. You don't need to know everything on the war, the main thing that you need to know is why Hitler got involved.Some things i can come up with are...
a) It was his chance to defeat communism there
b) He wanted to test out his new weapons and train his army for world war 2.
There are other reaons as well, i'm sure you can find them on the net or something.
I suggest you learn about the city "Guernica" as well, it's the city that Hitler bombed and Destructed.
Also remember that during this time, Mussolini and Hitler were allies. Their Enemies were Franco and the USSR.The Russians were basically supplying Spain with weapons.
The paper is gonna be on sources, so make sure you practice those as well.