Mechanical repairs and adjustments on the sewing machine normally need to be completed by a service technician. Special tools are normally needed to check and adjust the timing and position of sewing machine components.
Here are some general tips that help you resolve this problem:
Make sure that you have a fresh, sharp needle properly inserted.
Make sure that the bobbin is properly inserted and threaded.
Check the bobbin to see if it is overfilled. Remove some thread if it is completely full.
Here is a link for the owner's manual for your sewing machine:
Use the owner's manual to make sure that the machine is threaded and adjusted correctly for the fabric that you are sewing.
Here is a link for a service manual that describes the mechanical adjustments on this type of sewing machine:
This service manual is for models that are extremely similar to your model. If you are mechanically inclined, you may be able to use this manual to make some minor adjustments on your machine. Some adjustments would need the timing gauge that would be expensive and impractical to purchase for in-home adjustments. Most repairs and adjustments require the expertise and experience of a technician.
If you need more help, resubmit your question with additional details.