Are you putting the presser foot down before sewing? There is a lever in the back to lower the foot to the bed of the machine. That engages the tension mechanism so the stitches form properly.

If so, then it sounds like a threading issue. Take both the top thread and bobbin out, make sure the needle is all the way up in the clamp and that it's turned around the right way, rethread and try again. :)

The needle should have a flat side on the end away from the eye. On most machines the flat side goes toward the back of the machine but on my Juki the flat side goes toward the right side.

Donna