how do i cut a song on garageband to fit what i need?

After you add the audio file to a track, you need to trim the region to the part that you want.

See: Edit regions

Trim a region:

Tap the region, then drag the left or right edge of the frame. You can touch and hold the left or right edge of the frame to zoom in for more precise editing.If the region is currently looped, tap it again, then tap Trim before dragging the left or right edge.If multiple regions are selected, all of the selected regions are trimmed by the same amount.

You can shorten purple and blue regions on either their left or right edge, but you cannot lengthen them beyond their original length. You can lengthen green regions any amount to add silence, and can shorten them on their right edge, but cannot shorten them on their left edge past the first note in the region.

Regards

Léonie