If you scaling image over 100% it's going to start soften. It's as simple as that.
If you have a vector image, then you can turn on continuous rasterization and the image will remain sharp throughout all scale settings.
Rick is correct, but there are some features that can give better results than the default scaling:
By the way, it helps if you give a lot more information when asking a question. We have no idea what you've tried so far, and we don't even know what version of After Effects you're using.