I seriously need help!!!plz help me!?

ok, so im going out with this girl, and her and I seem to be doing ok, except that Im the jealous type,and im trying really hard not to show it, because Im really scared about getting cheated on...and to make things worse, this person I dnt even know with the user name sllade4u IMs me saying my girlfriend is cheating on me, and after a while, Im confused and dnt know if I should believe them or not....How do I stop being jealous and what do I do about this one guy??? what do I do with my gf? help me please... Im really nervous about this...please help me, and Thanks for your help!