I warn you, I'm pretty new to sewing still but these are the things I can think of.

Make sure you are using the right sized (class) bobbin. And, if it is the right class but came from a big chain store, it could be a dash too small. I bought some and wound up throwing them out. Try buying some from a smaller shop like a sewing machine sales shop. You'll know it is too small if it wiggles a lot while sewing.

Try using two differently coloured threads on a scrap-swatch of the fabric you are sewing with. Dark for the bobbin thread, light for the top thread, or what have you. then, look closely and see if the bottom thread shows up on the top or vice versa. Then, check your manual and adjust the tension accordingly. My manual has good pictures of just how it should look.

You can also just adjust the tension up or down until the problem corrects but I think this is harder to do and takes longer.

It can be a good idea to make sure that the thread is threaded properly and, even if it looks like it is, it never hurts to re-thread. Take it out and put it back in, making sure you don't skip anything.

Another idea is to make sure your needle is fresh. If you haven't changed needles since your last project, go ahead and change now or look under a magnifier and hold the needle to a flat surface to make sure it is straight. I never see anything this way so I just change often-ish.

The needle might not be in all the way. Loosen the screw and push up to make sure it is in, then tighten.

The last thing I can think of is that the needle could be the wrong size for the fabric. Double check the needle size and change if needed.

I hope that helps!!

J.

Good ideas above especially the needle. Also check to make sure you bobbin is feeding off in the right direction as you may need to turn it over.

Many times it can be that the needle is threaded wrong. Totally unthread the machine, then rethread following the manual.
Hope that helps, Kathy G in MI

I agree with others, also may sure you threading with needle up and using the proper size needles.
Have seen a few people use singer needles in some of the machines they are made a little longer
After pulling up bobbin thread pull a few inches of thread out to see if the threads are pulling evenly.
Check for burrs around the bobbin case.
Does your machine require a metal bobbin?
Let us know what works.
Is it a drop in bobbin or front loading type with metal bobbin case, might need tighten a little if it is the front loading.

Sometimes there is a small piece of thread or lint caught in the bobbin chase as it feeds the machine. You can carefully run a small pin through the chase under the spring and dislodge anything caught in there.

First change needle, totally rethread machine as first poster stated but I believe it is a tension problem... If tangling on bottom, then it is tension on machine if tangling on top then it is in the bobbin case and tiny screw needs to be tightened just a tiny bit. In your case turn tension to higher number gradually and keep trying the stitch forward and in reverse. Some machines may say in manual maybe around 3-5 but alot of machines need it even higher. Manuals are terrible these days... Mine for example is at almost 7! I know this post is old but so many need this information. Alot of people charged $80+ for repairs when it is just the tension! Not broken, just needs adjusting especially when machine is brand new! Hope this helps someone!!!