You're in luck....I've just graduated from the Masters degree course
If you're not quite sure what you want to do, then the certificate course would be a good place to start. If you do decide to do the Masters, then I'd suggest you brush up on your maths as you will be using quite a bit for the course. A brush up on your physics would be good as well. Those two particular courses in the certificate aren't too onerous but in the Masters course they do contain more maths/physics content. Those first two are rather basic level courses to start off with, but the (much) more intense courses do come later. The main thing is to not let the work get too far ahead of you and do the required readings. Get the textbook....it is expensive but it will be a big help. Get used to chasing down any number of research papers from journals and databases like arxiv.org. Try to read up on a bit more than you're given, but don't load yourself down with too much and if you start to feel you need a break, take it.
The lecture notes that you will be given just cover the basic outline of the course and point you to the references you will need to read. Most of the learning you will do will be by reading the textbook and doing examples from that as well. Learn your equations, but don't try to put them to memory. Best to write them down in a note book, so you can get a hold of them. The lecture notes will get you though the basics, but there is a lot more to learn and it'll be up to yourself to do this. I would also encourage you to buy other texts apart from your main textbook, especially if you do decide to continue further along than the certificate level. Whilst the textbook is comprehensive, more advanced work will need supplementary reading from specialised texts. Grab yourself some bibliography software, it's a big help when looking for journals papers and getting a "library" of them together. Makes it easier to find them.
If you have done algebra and calculus at Year 12 level and passed it OK, then so long as you can remember how to do it (!!!!
) you should do OK. If you feel you need some remedial work, then I would suggest you take it, just to brush up and to get back into the swing of things. If you've handled lots of maths in your previous background....probs and stats etc, then the other shouldn't be too hard for you. It is good to have a maths/physics/physical sciences background and they do prefer that for the Masters degree course, but you can do it without it....you don't necessarily have to take the purely astrophysics stream of the Masters degree. You can also do astronomical history and/or astronomy education as well. They allow for those that don't have the maths/physics/physical sciences background to complete the course.
Having a background in astronomy as an amateur will help, since you'll have that previous interest, but don't rest on your laurels because you have that background. I have the same background as an amateur and knew a great deal of what I was doing already, but you still need to be on top of your game even if you already know what's being taught. These are not necessarily easy courses, even with the less intense subjects. How much you do learn is up to you...as I said previously, the lectures just cover the basics that you need to know. To progress further you'll have to become more involved with the work. As with any course you do.
In any case, if you want/need more help and advice, I'm always here so all you need to do is ask