When I had trouble with this I checked back in my genchem book and that helped. There are 2 ways to do this. 1. you can look at the periodic table and see that orbitals fill in order as to their location on the table. For example, 4s comes before 3d, so 4s fills first (try to find a figure in the book that depicts this and you will understand it when you see it).
2. There is also the (n +l) formula. 4s=(4+0)=4 3d=(3+2)=5. 4s has lower energy, thus it fills first. n=principal quantum # and l is the subshell (s,p,d or f, =0,1,2,3)
hope that helps.
ps be aware that a typical mcat question will be about electron configuration, and they may give you a transition metal where the 3d orbital fills before the 4s orbital. regardless, the main thing you have to know is how many electrons the element has. then you can easily figure out the answer.