As someone who was about as predisposed to maths at school as Hannibal the Cannibal was to vegetarianism I feel more than qualified to chime up on this topic.
I remember asking this very same question before I started flying and recieved numurous(!) replies from all manner of peeps in the pro aviation field. These days because this sort of topic has been raised quite often those guys tend not to post on these matters anymore.
From memory, the useful areas to hit are:
Ratios
Percentages
Properties of Circles
Trigonometry
Algebra
Indices
Using a scientific calculator (square root key and squared key mainly although some have great arc to time conversion buttons which you'd do well to discover) is useful too.
I personally have not struggled too badly with the maths in ATPL. I left school with a 'D' grade in GCSE maths eight years ago. Rearranging formulae is probably the toughest part of the deal and to be honest with you I still ain't much good at that but with only four exams left to sit (which includes only one that has any maths involved) I'm not too concerned.
Most study notes from schools include the mathematical working in their notes when required but even so, it's a good idea to brush up so as not to waste time when you start the ATPL course.
VFE.