This is a problem that can only be resolved in a couple of ways
1. Take two images, one exposed for the sky, one exposed for the foreground and merge them together in Photoshop.
2. Wait for the light to shine on the subject and then the image will be exposed properly, but this is only ok if the light and location are in the right place ?
3. The most common, you will need a set of or a ND Grad's (neutral Density Graduated Filters) one part of the filter is dark and it graduates (hence the name) to clear. The darker part of the grad allows less light through it so will comabt the problem that you are having by reducing the brightness of the sky while allowing the darker foreground to be exposed properly.