Depending on your personality this may or may not be challenging, but I would simply advise to to look and feel like you've flown alone 100 times before, which you have (in effect - apart from the alone bit:-)

Don't keep telling people its your first solo flight; don't keep telling yourself this!

Then relax, and follow all the other excellent advice here about being organized, etc etc.

My first flight was also my first solo flight - when I was 25, from Gatwick-Minneapolis-San Francisco - Philadelphia - Gatwick. Looking back 25 years, I'm more surprised at how much of San Francisco I explored alone and at all hours, than by the fact that I navigated immigration and Minneapolis airport.