Weak to the core!

Whatever strength training exercise you are currently doing keep doing it. It will only get better over time. Here's a few: http://blog.myfitnesspal.com/the-30-day-strength-building-challenge-for-beginners/

Planks. Ugh they suck, but they work. Time yourself and see how long you can go, then next time go a little longer, etc.

If you're really weak starting out, it can help to use a fitness ball. My PT recommended most of the exercises in this vid to me after my back injury- They look pretty easy but they aren't. Once you master the positions, it's easy to up the reps, and then 'graduate' to planks and what not.
https://www.youtube.com/watch?v=U3LWxSXwNX4