There are two basic ways to make a fixed-wheel jogging stroller turn where you want to go. Both will feel quite different from turning a swivel-wheel stroller. With a swivel front wheel, one pushes into the front wheel(s) so that it will grip and ground and turn. With the fixed-wheel alternative, you will either
1. lift the front wheel and pivot on the back wheels or
2. torque the stroller to turn without lifting any wheels off the ground.
If you need to make a big turn, such as turning a corner, you will need to push down on the handlebar to lift the front wheel off the ground and pivot on the back wheels. The easiest way to do this is to bring your body in close to the handlebar to take advantage of your core strength and possibly even the weight of your upper body. For big turns or with heavy kids, you may find it helpful to lean with your outside forearm on the handlebar to push it down. Make sure that your kids are leaning all the way back into their seats as well.
For smaller or more gradual turns, it is not always necessary to turn with the front wheel off the ground. I can best describe this option by comparing it to turning a boat by heeling in the opposite direction; heel right to turn left and vice versa. Of course, if you are unfamiliar with sailing, that is not particularly helpful :) This option is more of a lateral push that will torque the front wheel in the direction you want to go. This is ideal if you are making gradual turns along a curvy path or around obstructions that you can see well in advance. Note: This technique will be much more effective when you are running that if you are trying to walk.
If you find this very difficult, there are a few key muscle groups that can help. That's Part II.