First, I would urge you to consider that your machine may be out of adjustment. Let's eliminate that as a possibility first.
Go back to square one:
1) Un-thread the machine, both top & bobbin.
1a) Remove the needle plate and clean out all the lint that's in there. Use a small artist paint brush to help you.
1b) Oil your machine, following the directions in your manual. Use only sewing machine or Tri flow oil.
DO NOT use 3 in one, WD 40 or the like.
2) Put in a new needle 90/14 size. Make sure it is put in properly. Consult your manual if you do not know which direction the flat side of your needle faces.
3) Wind a bobbin with the same thread as you are using in the top. Use a good quality all purpose thread for practice. Make sure you have the right bobbin for your machine.
4) Re-thread the machine. Be sure your presser foot is in the UP position for threading.
4) Make sure you are using the single hole needle plate if you have one and that your needle is positioned exactly over it.
5) Using your straight stitch foot, feed dogs UP, and 2 layers of cotton fabric, sew a few lines of straight stitches at 12 stitches per inch.
6) Adjust your tension so that it looks perfect with no puckers. (Be sure your presser foot is DOWN whenever you adjust your tension.)If your fabric puckers, loosen the pressure on the presser foot (if your machine has this adjustment).
Number your lines of stitches and make notes of any changes you make. Only change one thing at a time.
If you cannot get the tension right at this point, then a service call is in your future.
Once the tension is properly balanced for straight stitching, make a small practice sample sandwich about 12 inches square. Hand baste in rows about 2-3 inches apart. Baste a grid onto this piece.
Using a pencil or marker of your choice, draw lines about 1 inch apart. Using each foot you have, starting with your SS foot, stitch on the lines. Make adjustments as necessary to get the best straight lines you can. Make one adjustment at a time and make notes as before. Do this with each foot that you have: straight stitch, zipper, SID foot, quarter inch foot, and finally, darning foot.
Make more than one sample if you need to. Be sure to make accurate notes. Keep your samples and notes handy for reference.
Now, using the foot that gave you the best results in your test, practice stitching a grid on a practice sample.
When you are satisfied with these results, go to Leah Day's website for more free motion tutorials. Her website is free, and you can watch videos all day.
I'd also suggest practicing free motion with paper and an un-threaded machine.
These are all exercises that I teach in my machine quilting classes. If you can find a class close to you, that would really open you up.
Best of luck,
Stephanie