When trimming a chicken's toenail, its important to note that you don't want to trim much off. You want to avoid hitting the quick, which is where the blood flow is in the nail, as shown in this illustration from http://www.lafebervet.com/. You can cut anywhere to the left of the imaginary yellow line.
Trimming beaks can be done if you notice that it is affecting the way that a chicken eats, or if you notice that a chicken is losing weight. After trimming a beak, you can smooth out the sides with a nail file. Just be sure to give you chicken lots of breaks to keep them from getting stressed.
Hope these tips help with your beak or nail trimming endeavors. If you have any questions, please comment below!