Any rack made by a reputable company will do fine.
Topeak racks are affordable and strong. The "explorer" is a $35 tubular rack that offers great bang for the buck.

Be sure to check your bike for rack mounting options. I'll assume the rear dropouts (where the rear wheel mounts) have rack eyelets, but also look to see if the seatstays (the tubes that run from the seatpost down to the rear axle) have mounting hardware for the rack. If not, no worries, you can buy hardware that accommodates this need (rubber-covered metal straps that wrap around the tubes). Just be sure to get the appropriate sized strap for your needs.

Also, if your bike has disk brakes, your rack will probably need to be a 'disk brake specific' rack.

Enjoy!