I have a bitter taste on the tip of my tongue and on my lips. What can it be?

Many possibilities. At age 19, most likely source is a vitamin deficiency. Could be a medication side effect, an after effect of gerd/heartburn or an allergic reaction to a lotion, lip balm or the like. See your dentist for a diagnosis.
Nutrtional deficienc. Some times lack of certain minerals/ vitamin can give peculiar tastes in mouth. Also chronic sinusitis can also change taste buds.