I'm not familiar with the WJEC spec, but you could read over the spec to get familiar with what kind of content you're expected to learn to start off.
The youtube channel craig n dave have lots of short and easy to understand videos that cover the AQA and OCR a level specs, which will probably have a lot of overlap with the WJEC content, so those are probably worth checking out. Did you take GCSE compsci? If not, it's probably also worth looking over resources for GCSE as well.
I hope this is useful for you!