if your bottom thread is pulling up on the top the top tension is too tight, if the top thread is showing on the back the bobbin tension is too tight. start with adjusting the top tension just a little at a time and use a practice sandwich which has the same batting/fabric weights trying a line at a time until you have your tension even top and bottom. it is easier to adjust top tension but on occassion the bobbin tension does have to be adjusted. are the threads different weights? such as 50 wt in the top and 100 wt in the bottom? sometimes this makes the difference, but it is fine to use 2 different weights you just have to adjust the tension to accomodate it.