I agree with sbd-5, hold a little back stick.

Turn your radio on and look at the elevator. With that size engine in your trainer, I'll bet you have some down elevator trimmed in to keep the plane from climbing at altitude. If you do, here's what's happening.

As your plane gains speed, the down elevator tried to raise the tail, forcing down on the nose gear. There is always some slop in the linkage, the strut and the flew in the wheel. As you put in rudder, the gear tends to flop or lean to one side. When you correct, it leans to the other side, giving you the zig-zag motion.

You want to get the weight off the nose gear and let the rudder steer the plane. Your plane ought to take off in about 20 feet. Hold in about half, 1/2, up elevator and add power. There is a tendency to move both hands forward when you add power, releasing the up elevator, so be careful. Once you gain speed, the nose will come up and in another few feet, you'll be airborne.

In addition, don't hold in rudder, blip it for corrections.

If you are flying at full power or near to it, try 2 popsicle sticks under the trailing edge of the wing. This will allow you to reduce the down elevator needed to hold level flight without climbing. You would like to raise the trailing edge enough so your elevator trim is level at the power setting you normally use.