I can write their molecular geometry but I don't know how exactly I'm supposed to compare them together ... for instance, in CH2F2 , 2 F can be on any side of tetrahedral molecule!

That is right it can be on any two sides but they are both on the same side no matter the orientation so the highly electronegative Fs pull electrons towards them making that side partially negtive.

However in CF4 , the 4 Fs are even disperersed around the central C and because they pull equally cancel each other out so the molecule is not polar.

If you have a model kit, build out all four molecules and really look at them. If the Fs pull electrons in their direcitons , where is the electron cloud pulled.

One way I used to describe this is to think about it as a tug of war around the central atom. If everyone pulled with the same strength towards the more electronegative atoms, in this case, flourine. Where would the partial negative charge end up.

That is right it can be on any two sides but they are both on the same side no matter the orientation so the highly electronegative Fs pull electrons towards them making that side partially negtive.

However in CF4 , the 4 Fs are even disperersed around the central C and because they pull equally cancel each other out so the molecule is not polar.

If you have a model kit, build out all four molecules and really look at them. If the Fs pull electrons in their direcitons , where is the electron cloud pulled.

One way I used to describe this is to think about it as a tug of war around the central atom. If everyone pulled with the same strength towards the more electronegative atoms, in this case, flourine. Where would the partial negative charge end up.