i would suggest using tsr personal statement building or making a plan in bullet points (similar to what was suggested above or google what to include in a psychology ps to get ideas) and one you have that start adding information to the bullet points - adding information in bit by bit, then redraft it into more of a coherent piece (i use this for writing essays too)
also i would suggest showing it to as many people as you can - teacher, parents, family friends, friends, everybody you know! and using all their feedback to redraft and then show it to some people again
good luck!