I just finished my first scarf and the edges are rolled under and will not lay flat. What am I doing wrong?
I don't think you've done anything wrong. Rolled edge is a very common problem when knitting with Stockinette st. That's why garter st border is recommended for border and edging to help prevent rolling.
You can make your scarf look better with less rolled edges by blocking it. This is the way I usually block my piece;
You'll need some towels. May be 3 big ones.
1. Fill a sink with half way with water (room temperature). 2. Fold scarf and push it down under water until wet. 2. Drain the water from the sink and squeeze out as much water as you can from the scarf. 3. Lay a towel on flat surface and transfer the wet scarf to the towel. 4. Spread the scarf over the towel lengthwise. 5. Start rolling one end of the towel over the scarf until reaching the other end. 6. Press the rolled towel with your hands to get rid of excess water. 7. Place new dry towel(s) in the area where you can leave it to dry for a day or two with disturbing. 8. Lay the scarf over the new dry towel(s) and smooth it with your hand. 9. You can use pins to secure the scarf in place if needed.
Good luck with the blocking and I hope this helps and thank you for submitting your question.