Making stuff transparent isnt so hard once you know how.
1. Making a layer from the start transperant:
Create a new image and tick the plus from the Advanced Options.
Select the option bar from 'Fill with:' and select 'Transperacy'.
2. You can make the layer transparent by right-clicking the relevant layer and selecting 'Add Alpha Channel'.
The layer name should be un-bolded. Now erase a random part and you should see this:
That is transperacy.
VOILA!!!