I have done this in the past by first making sure the Att phone is unlocked ( https://www.att.com/deviceunlock/#/ )… which is a good idea even if not changing carriers. Then go to to straight talk site to verify compatibility
( https://www.straighttalk.com/wps/portal/home/h/a/options/byop/!ut/p/a1/lc9BDoIwFATQs3CCDiCl2wKhraAgWMRuDCvTRNGF8fyCcaNR1L-b5E0ynxjSEtN3V7vvLvbUd4cxG7qLWK5Vw6VXzWIfPPN4XYgUiOkAthOABb_1E8wX_moAkZAJ1JqVdUY1IPBnv6IugypzkSZu7oOF3_obYibJ-MEdTE18Bm82vAAZBgOQcS3CJYrUewB8OA5yPmrdwirLHecGR_lY5Q!!/#GUIDEDLANDING )…
Go to walmart or purchase your straight talk Sim kit online :)