Im going to second rgh's advice.
I also got 45 Ds, so it is doable.
Definitely start assignments straightaway. You might get away with leaving them a week in the very beginning, but as the course progresses the assignments take longer. We were getting three or four at a time with a fortnight turn around to submission, and the next lot.
It is imperative that you check the assessment criteria for each assignment (which will be included). The course is intensive, and youre being taught for each assignment. So what your tutors teach you NEEDS to be in your work, no matter how basic it seems to be in the beginning.
You can also pick up easy quality marks for presentation and referencing, so get them. For each module, the first assignments were the easiest ones to get Ds on, because they're basically testing how well you've understood the introductory concepts. Work hard on these, easiest Ds you'll get!
I was the opposite way round to rgh, first two months we were eased in. Then it got hectic!!!