For Michonne: A dirty tank top to show off your extremely toned arms, a leather vest if you have one, a pair of olive-colored skinny jeans, and a bandana to put over your forehead to keep all the sweat you'll work up from slaying walkers out of your eyes. Grab a fake katana to complete the look.
For Rick: If you're going for an early version of Rick, a dirty, light brown cop uniform will do. In later seasons, he wears jeans, a plain white T-shirt, and a sheepskin jacket. Splatter it all in blood, make sure your face is covered in scruff, and you're good to go.