Planking uses (what feels like) every muscle group, but centers on your core. If you find you are having pain in your lower abdomen (under your belly button), try planking on your knees instead until you can build up the strength to do more. It's important that there are no butts in the air and no slouching butts either. You want to create a flat line as showed in above image. You can't say you don't have 30 seconds a day to do this one. No excuses.
Starting out simple allows you to gauge your own abilities. Remember not to hyper extend your hips OR slouch those hips. If you are having knee pain, try placing your top foot slightly behind the other to offset some of that pressure.
If your a beginner to this exercise, Eat2Live recommends starting with your knees in a bent position, not straight.
As you develop more flexibility and strength, this exercise can still come with you and help strengthen more than just your core and back.
No matter where you are starting, if you don't quit, you WILL reach your goal. Eat2Live recommends trying these daily. The entire workout last less than 3 minutes, most people spend more time than that staring at their iPhones on the toilet. You can do this.