Side stitches are actually pretty common in new runners. It all relates to your diapraghm. Just like other muscles in your body, these muscles have to adjust to the new exercise routine as well. The cramping is caused by exerting yourself more than you are used to and demanding your body to pump more oxygen.
So what do you do? You practice breathing from your belly, rather than just shallow breaths. Also, keep training, working on posture, especially on hilly terrain, which can be a major source of side stitch pain. And if you do get a cramp while running, you can purse your lips and exhale to relieve pressure, putting your hand over the area.
The big thing is to not let this stop you. It is a completely normal thing for a new runner who is pushing his/herself to improve his/her physical fitness. Just like your other muscles, your diapraghm will build up and pretty soon, those pains will be a thing of the past.
Hope this helps, and good luck with your 5k! I still remember the excitement of running my first one, and my only warning is they can get kind of addicting! :-) Anyways, just keep going and I'm sure you'll do awesome!