You didn't explain the symptoms. Is the thread bunching, looping, knotting under the fabric? Is the bobbin thread rising to the top of the fabric?
If it is bunching under the fabric, it is probably what is called thread nesting or bird nesting. There are several fixes, sometimes more than one fix is required to resolve the issue.
Be sure to clean under the needle plate, around the bobbin, and the feed dogs. If the machine has instructions for oiling, follow the directions using a fresh, good quality sewing machine oil and apply only 1-2 drops each spot.
Install a brand new needle. Make sure you are using a needle compatible with the thread and fabric being used. All About Needles
Best practice is to use the same thread in both the upper and bobbin. Also, avoid old or bargain bin thread. Spend an extra couple of dollars to reduce frustration and increase success with your machine. If you wish, you may use two different colors of thread to test so you can easily determine what is happening with the tension.
Remove the thread from the machine. RAISE the presser foot and rethread from the beginning--the thread needs to be seated firmly in the tension disk. Verify that the correct thread path is followed (consult the owner's manual).
Set the upper tension to the midway point between high & low numbers (this is the factory standard and is a good place to start, but every machine is different and may need adjustment).
Confirm that the bobbin is installed in the bobbin case (turning the correct direction) and that the bobbin case is threaded properly.
Before beginning to stitch, hand rotate the handwheel one rotation and pull the bobbin thread to the top of the fabric. Then, gently hold the thread tails in the left hand and slowly begin to stitch.
If thread is bunching, looping under the fabric, the upper tension is too loose. Tighten it and retest. On most machines, the lower the tension number, the looser the tension.
If the bobbin thread is rising to the top of the fabric, the upper tension is too tight. Loosen the upper tension and retest.
Keep adjusting and testing until it is where you want it. Generally, tension is correct when both threads (upper & bobbin) meet in the middle of the fabric. FWIW, tension is static and should be tested and readjusted every time a new project is begun. The needle, thread, and fabric will affect the tension, so it is important to get comfortable adjusting the tension.
Also found the following reviews. Appears that some users have had tension issues with their CG550.
Amazon com Customer Reviews Singer CG 550 10 Stitch Commercial Grade...