The biggest thing without know anything else, is to know what kinda of shape you are in and know your personal limits.  Make sure that you consult with a physican prior to any new training, and continually train for your event.  If you have never run before there are programs called "Couch to 5k" designed for new runners, but your 5k is in 5 days, which isn't enough time.  Actually with the amount of time you have, I would do the run and enjoy it and then work hard to get better on you next 5k.  Good luck on your run.

I would say just do your best and do not push too hard on your first 5k.  It is okay to walk/slow jog or whatever is comfortable.  Most likely, you will have a wonderful experience and become addicted to races...looking forward to the next opportunity.  A lot of events are for charity, so you will be improving your health and making a difference.  Also, you will probably meet some wonderful and encouraging people to inspire you on forward.  I would encourage you to talk with other runners before and after the race. The overwhelming majority are kind, generous and helpful advocates, willing to share insights and advice. 

Wishing you a memorable experience and many more in future!