I offer a few ideas.
Re-thread with a quality thread such as Guterman, Drima, (Coats/Silko cotton only if you are using 100% cotton fabric). Not Moon thread made by Coats that's only suitable for an over locker.
Check the needle is in the correct way round, flat side to the back. Exceptions to this rule are vintage singer and industrial machines.
Load the bobbin correctly, there will probably be a diagram to show you. Close the bobbin cover.
Foot still up and no fabric, turn the balance wheel towards you. This will send the needle down into the bobbin case. Keep turning until the needle rises to its highest point. A loop of thread should have been brought up from the bobbin, pull it free and pull top thread and bobbin thread out to the back and to the left a good few inches.
You are now ready to put your fabric under the presser foot. Lower the presser foot and start sewing. For straight stitch width 0 length start with 3 and adjust to what suits you, upper tension probably around 4 adjust as necessary. Don't adjust the bobbin tension. When you start sewing hold the threads out the way so they don't get tangled up and cause a big birds nest of a knot.
When you finish your stitching, raise the needle to the highest position (again using the balance wheel) before raising the presser foot. Pull your work away so as not to break the needle. Either cut threads with scissors or use the thread cutter on the machine.
If you do exactly as I say and still can't get a good stitch then I think you must have a dud machine.
Hope it works for you.