This is because of default padding and margin, so you can select div and set margin and padding to 0, or value you need.
Most developer do "resets" by selecting all elements, or root or elements thay need..
* {
margin: 0;
padding: 0;
}
This is more about resets, with some example http://web.simmons.edu/~grovesd/comm244/notes/week4/css-reset
