Hi,
You did OK, thats how you learn about the machine. Sounds like you loosened up too much on the top tension--the knob on the left. Put the presser foot down and tighten and/ or loosen until the thread tension feels right by pulling the thread through the tension disks. Try sewing and adjusting until your happy with the stitch.
The stitch length knob is on the right on the pillar. Turn counter clockwise to shorten length of stitch. Don't have to adjust tension to change stitch length.
Here's a copy of manual which explains process much better than I did.
http://www.singerco.com/uploads/down..._127-128-3.pdf
Good Luck
Jon