PilotINOP as a former ATPL theory instructor I was alarmed at the standard of maths particularly from UK students, some had to get a calculator to work out 10% of 200.
However you don't need to be a maths genius (no differential calculus or quadratic equations) but these are some of the key areas:-
1. Being able to re-arrange basic formula eg. Distance = Speed * Time.
2. Cross multiplication of formula
3. Percentages - increase/decrease and back working net
4. Being able to interpolate tables and graphs
5. Pythagoras - SOHCAHTOA
6. Understand that your navigational computer (CRP-5) has a logarithmic scale
Most of the time any calculations will be done on a scientific calculator or CRP-5 in a classroom but when out flying and also recently introduced into ATPLs you are going to have to use some simple mental arithmetic.
Even when using a calculator get into the habit of gross error checking your answer (is the answer reasonable & is the decimal point in the correct place).
You shall also need to know some basic physics such as the gas laws.
You could start with the BBC bitesize but Bristol Ground School (BGS), CATS & Padpilot (there may be more) offer a Maths & Physics for Pilots book/download which will be a more focused option.
All the best RichardH.