Mickey2 gave you very good information. I have a 15-91 that in finally got sewing well just last month. The first two things I would highly recommend you do are 1)make sure the needle is in correctly (flat side to the left) and 2)bobbin case is threaded correctly (thread should come off the top of the bobbin in a counter clock wise direction).
For thick seams you will need something behind the foot to help keep it level like Mickey2 mentioned. I use a thing called...a Jean a ma Jig (or something like that).
I had so much trouble w/my machine for more than a year and it was ALL because my needle was in wrong. Maybe yours will be an easy fix too. I am now having so much fun sewing again because my machine works properly.