i want to be a dentist too.
if you look at most UNI's they actually only require chemistry and biology.
however they tend to recommend taking either maths or physics
(further maths is not counted)
as you can see you are left with on option of any subject.
this is a free pick so you should do psychology as its easier compared to other subjects.
as long as you have A GOOD STRONG grade in maths, biology, chemistry. then you will get in successfully in any dentistry course
hope this helped