Congrats on joining the tri community! How long have you been riding? From your comments, I'm guessing it's a fairly new activity to you. It takes time to learn your personal "sweet spot" for gearing, and for changing gears to become totally second nature. Do you have a bike computer that shows you your cadence? For most folks, somewhere in the 85-95rpm range works best. You'll slow down a bit on steep hills, but for the most part, that's a good range to shoot for. You can fine tune from there. If you are "mashing" (riding in a harder gear and pushing a slower cadence), you are overworking your legs and your run will suffer. Tri is not 3 sports back to back to back. It's one sport. You can't have a GREAT bike and a lousy run. That means you had a lousy bike. The sport is complicated and takes time (years) to really master. For your first race, concentrate on having fun and enjoying the experience, and then move on from there!
Riding with others is a great suggestion - that will likely push you a tad harder than you'd otherwise push yourself, and over time, you'll improve.
As far as nutrition goes, certainly find what works for you, but for a sprint race, nutrition isn't a consideration - it's simply too short a race. You really don't need anything at all on the course - MAYBE something mid-bike, but only if you know you need it. But find out what works for you for breakfast - you'll certainly need to fuel up solidly in the morning of the race, but with something you know will sit well.
Good luck!
