What to do about low testosterone levels?

Nothing unless . . . Low testosterone is just a number. We don't treat numbers (aside from blood pressure, cholesterol & glucose). Instead, we treat patients. So if you're tired, fatigued, fat w/o muscle despite diet & exercise, no sex drive w/erectile dysfunction, can't think clearly, and depressed, you might have hypogonadism which might be cured by increasing your t (beware some rx might affect sperm production).
Diagnose and treat. Your doctor should test to find out why testosterone is low --- is the pituitary gland or the testes not working? If the pituitary ('master gland') in the brain is under-performing, further tests and treatments are needed since many hormones may be involved. Treatment for low t include testosterone solution or gel or patch (all via skin) or mouth trouches or injected solution or implanted pellets.
See doctor. See urologist or endocrinologist to diagnose and treat low t.
Depends on age. If you're not worried about fertility in the future you can replace with pellets, topicals (androgen etc), injections or patches. If fertility is a concern there are some "off-label" treatments that can preserve fertility. For more: www.Peedoc.Com @thepeedoc.
Discuss with your Dr. You must have had your testosterone level checked for symptoms of low libido, ed or lack of energey. So obviously you want traetment. Discuss with your internist, who may be comfortable prescribing testosterone gel or injections. Your dr. May suggest seeing a urologist or endocrinologist. Always best to go thru your primary care dr.