Thread bunching under the fabric
A nest of thread underneath is almost always the top thread, not the bobbin. Rethread with the presser foot raised.
Fix common stitch problems
Work through the causes in order. The first two or three fix most machines.
Skipped stitches
The needle comes first: new, the right size and type, the right way round, pushed fully up. Then threading, lint and foot pressure.
Upper thread keeps breaking
Rethread, check the spool and pin, ease the tension, fit a larger needle, then look for a burr on the plate, foot or hook.
Setting top and bobbin tension
How to read a stitch, why rethreading comes before the dial, and the starting numbers each manufacturer gives.
Needle not picking up the bobbin thread
If no loop comes up when you turn the handwheel, the hook never caught the top thread. Needle, bobbin direction, then threading.