This tutorial gives you a basic look at the shapes and anatomy needed for drawing realistic mouths. Make sure to watch my other tutorials on facial features!
In this video, I am using a Prismacolor Ebony drafting pencil, but you can use any kind of pencil - plain old HB pencils work great! Try to stay within the HB-2B range for general sketching. Too hard of a graphite (the higher the H, the harder the graphite) is really light and can indent the paper easily if you press too hard. The higher the B, the softer the graphite, which will create darker marks!
*This video is also on my classroom channel, DGFMSArt