If your stitch isn't nice, than most of the time, your thread has not been threaded correctly.
Sometimes the needle is not sharp anymore, damaged or crooked.
Always start with a new needle! ( order new needles here )
Rethread upper and lower thread again.
Hint: if the tension on the right side of the fabric isn't good, than most of the time the problem lies in the bobbin thread. If the tension on the wrong side of the fabric isn't right, than the problem is most of the time the upper thread: not threaded correctly.
If threading didn't help, try this:
Remove needle, stichplate (depending on the machine, is not possible for some machines), foot, bobbin and bobbincase.
Clean thoroughly the bobbin area with a vacuumcleaner or compressor. Oil the machine according the manual (some machines don't need oiling!)
Insert bobbincase, newly wind bobbin, use a new needle (adapted to your fabric), and foot. Use the same quality thead as upper and lower thread. Thread upper thread (hint: for some machines it is easier to thread with the upper tension turned to 0, but don't forget to put it back to normal before starting to sew!)
Test machine