Analysing sources is the part of A-Level History people tend to find most difficult. So you're not alone!
Unlike with essay exams, it's pretty much impossible to question-spot and prepare an answer in advance to be replicated in the exam. But there's plenty of techniques you can use to get yourself ready.
The key is to give yourself time in the exam to read the sources thoroughly. It's stressful under timed conditions, and it's easy to jump straight in and start writing. But you will only understand the message of the source, its nuances, and start to forge comparisons with other sources if you plan in the exam. I'd leave at least 20-30% of the exam for this.
First, work out what the source is saying, its 'message'. Underline parts that are important. Second, once you've read through the sources the question asks you to address, read them again. This time, try and notice common themes between the sources, and link them to the question asked. Or, if the sources are radically different, note that too. Cross-referencing is what gets you top marks. Denote these themes by number, or by colour with highlighters.
When you come to writing your answer, examiners much prefer you to structure an essay by theme than by source, as it proves you're thinking about the question. Planning in this way will help you achieve this.
Whilst doing this, it's really important to consider the source's provenance, or who is writing or drawing it. You'll have encountered this at GCSE, but at A-Level you need to be more sophisticated in how you approach this. Think about the author's motives, as at GCSE. But think also about the year of the source, what might have been influencing them at this time. This is where your contextual knowledge is crucial. Then, most importantly, when you read the source for the second time, analyse whether this context has had any bearing on the source itself, on what is actually written.
You will only master all this if you have a go at lots of past papers. Even though the sources in the exam you sit will be different, the general techniques you develop are applicable. Contextual knowledge is important, but be careful not to over-burden yourself with facts, because most marks will come from source analysis. So focus instead on the sources and on analysis, and on structuring your timings in an exam very different to the ones you sat at GCSE. Good luck!