Train.

Train like you race... race like you train.

If you train and take this off in little bites, you will see that you can accomplish this goal.

Yes, it's scary. And even with proper training, you will still be scared until you jump in the water and start swimming. But, you will finish, because you have trained.

Make a training plan. Do bricks (bike/run combos), and stick to it.

Also, www.beginnertriathlete.com, is a wealth of information has had a "couch to sprint tri" training program you can follow.

Best of luck to you!