Career...why is it so tough?

I'm at this juncture in my life where i've to decide what career to step into. I'm at the threshold of the corporate world and boy it does not look too good from here. Profs say, 'not to worry too much about the job u first take coz ur sure ur not gonna stick to it for long', but i personally believe that the first job u take would essentially determine your careeer path. What i'm trying to say is that if i'm going to enter sales then that's the area where i would be focusing my skills and training on. I hope its not going to be too tough for me to shift towards marketing later on. Frankly i'm still figuring out where i'm heading towards. Not too sure if i wanna do sales, marketing, PR, Corporate Communication, etc. I seem to fit into a gamuet of roles. That's not a good sign....is it?