I always have an error in any PHP script in the "start_session()" line...PLEASE HELP!

I always have an error in any PHP script in the "start_session()" line. Could it be because I have forgotten to enable some option when I set up PHP on my server or is it that I haven't set up the session correctly. I don't think It's something I did because I normally download free scripts that come configured and I just change the hostnames and stuff... PLEASE HELP ME WITH THIS!!! <IMG SRC="smilies/confused.gif" border="0">

Try posting the exact error message as this may help when figuring out what the problem is.