I'm not familiar with that particular sewing machine, but here are some general ideas.
To check your bobbin tension, load the thread into the bobbin case, then hold the tail of thread and let go of the case. It should not move when you just hold it. If you give a sharp flick of your wrist, the thread should unspool and the case should move down, then stop. If it's too loose, the case will continue to fall, or may not even require the flick to get it moving.
If the top thread is pulled down to the bottom side of the fabric (or into the bobbin area) when you sew, it means the needle tension is too loose or the bobbin tension is too tight. You might be making it worse by tightening the bobbin tension. Test this by loading two different colors of thread in the top and bobbin. This way you'll be able to tell which one is causing the problem.
Remove all of the thread - top and bobbin. Look inside the space where the bobbin case fits (in the machine). Get right down there - you're looking for threads that may be caught in the bobbin area. Remove the needle plate if possible and check from the top as well. The tiniest little 1/4" piece of thread caught in the bobbin area can cause a terrible mess. (Been there.)
Was the bobbin wound and inserted correctly? If the thread on the bobbin is squishy instead of firm, the bobbin wasn't wound properly. Make sure you insert the bobbin into the bobbin case so it unspools in the correct direction. Most machines with removable bobbin cases should unspool clockwise as you're looking at the lever side of the bobbin case.
Has it been oiled recently, or consistently? If it hasn't, try oiling it lightly in the designated spots.
Is the needle in right? I know it sounds silly, but if the needle is turned wrong, it won't sew properly. Look at the needle closely. There's a groove above the eye where the thread lays. The groove should be on the same side as the bobbin. If the bobbin loads on the front, the groove should be in the front. If it loads on the left, the groove should be on the left.
Is the needle the right size for the thread and fabric you're using? If the needle is too small for the thread, the thread will shred. Also make sure the needle is not dull. As someone in a group I belong to said, we'll go out and spend $8 a yard on fabric, and then complain about changing a $1 needle.
Was it threaded properly? Make sure the presser foot is UP when you thread it - this affects the top tension, and can cause looping on the bottom. Also make sure the thread is decent quality and not too old. Thread will dry out and become brittle. Cheap thread has slubs that can catch in the needle.
I assume you already checked the manual, if you have one. If you don't have one, try checking online - there are many places that sell manuals, and it's a good investment.