You won't need to as this will be provided in class, but if you want to practise, this might be advantageous at home. It's unlikely to be a requirement of the course though.
I've got Instagram, do I really need to know about Photoshop?
While Instagram is a great photo editing and sharing app, it's very basic compared to Photoshop. If you simply want to make your photos look pretty and share them with friends then it's probably enough, but if you're hoping to work in an industry that requires photo editing skills, you'll need to be well versed in Photoshop.
Do I have to know how to use a Mac?
Not necessarily, as Photoshop is available for PC as well. However, this is often the preferred desktop computer for many creatives, so if you're worried about Macs, it's worth asking the course provider before booking onto a course.
I'm no photographer - can I still learn to use Photoshop?
Yes - you'll probably find yourself editing other people's photos as practice anyway. A large part of Photoshop is improving photos, so if you're not very good at taking them, this could even be an advantage.