What should i do -.-?

Well theres this guy that likes me and i dont like him.He asked me out and i said no i dont like you.And later on he started following me and looking at me and well stalking me.My friend Alyssa goes to math club with him after school and math club was in my homeroom.Later on Martin (the guy that likes me) started acting really wierd,He sat in my desk and he started touching my nametag and my desk and then all the things that i had inside my desk for example my compisition book,my notebook and all the stuff in my desk.He started looking throught them and all that.The next day Alyssa told me all the wierd stuff he did and i COMPLETELY freaked out!Then the bell rang and it was time for homeroom.I sat in my desk and i saw something wierd.It said f*ck you ***** on my desk.I totally knew it was Martin,He did that because i said no to going out with him and he got mad.Today he kept on staring me and i heard him telling a friend that he knows that i love him.That is not true!And then he kept on looking at me and he kept on smiling at me.Then i said stop stalking me and looking at me!Then he just walked away and smiled.What should i do?He is starting to be a real problem,Answers are apperciated,thanks ^^