/diy/ - Do It Yourself

I'm brand new to sewing and I'm making some curtains.

When I try to sew a hem, the foot of the machine drags on the top piece of fabric, and the machine pushes the bottom bit of fabric forward from the bottom, so after a couple of feet the fabric gets misaligned.
It's straight when I pin it, but as it goes through the machine it gets pushed, and the part that's folded over ends up at an angle.

How do I keep the folded part aligned while sewing it?