How can I get rid of or relieve back pain due to spondylolysis?

Several ways. 90% of the time without surgery. Assuming this is the congenital or isthmic type most respond to activity modification, core muscular strengthening and flexibility , medication and occasional use of a back support. Sometimes injections can be of benefit . If no improvement or recurrent issue, surgery is an option.
Good question. Spondylolysis which is essentially a congenital or acquired fracture of the pars (an area connecting the front and back of the spine) can be difficult to treat, pain is typically multifactorial and worse when accompanied by misalignment (spondylolisthesis). Treamtment may include pt, interventions (epidural injections) and surgery. I would find a mckenzie physical therapist.