Building shoulder & lower back strength will help a lot. Exercises like shoulder press, lateral raises, shrugs, and deadlifts will help you do so. However, the main way to be able to do the plank is to simply start doing it regularly, no matter your body type. Even if you can only do it for ten seconds at a time, it's better than nothing, right? Also, if you've enrolled in a gym, asking a trainer to help you with the correct form will also help. Over time, the duration of your planks will increase.