Assuming you are studying at A level:
The peak with the largest m/z value is the molecular peak. This is the largest / heaviest possible fragment from the sample molecule as only an electron has been lost to form this ion fragment.
All other fragments created by the MS will have lower mass, therefore lower m/z value.
(Note, it is possible to get a very tiny peak just higher than the molecular ion peak due to isotopes of elements present in the molecule. The genuine molecular ion peak will not be very tiny, but will be easy to see on the mass spectrum).