How do I find out if its cancer? I'm 31 and I found a new mole on the back of my leg. It looks like a regular mole. Could it be cancer. Or could it be just anouther mole?

Biopsy. The only way to tell if a mole is malignant is to have it biopsied. This is most important for any skin lesion that is changing in any way...Size, shape , color, bleeding, pain etc.
Possibly. I would suggest that you have the mole evaluated by a dermatologist. It is certainly possible to develop new moles throughout your life, but if you have a history of sun exposure and sunburns, it is safer to have a professional check. Typically an examination with a dermatoscope may be done. If there is a concern, a biopsy may be suggested.
Any . Any moles that appear after the age of 20 should be assessed by a dermatologist to ensure they are not cancerous.