Can a possible pinched nerve or bad posture be causing my headaches? They are usually in one spot of my head and its more like a pressure feeling. Thx

Possibly. Sometimes a problem with the occipital nerve can cause pain in the back of the head. Posture is probably not related. However this type of discomfort is nonspecific, meaning there are lots of possible causes. Probably should be checked by a neurologist.
Usually muscle spasm. Most tension headaches are caused by sore, tight muscles in the base of the neck. Look up "trigger point" on wikipedia to get more information. Often the underlying problem is anxiety and poor stress management skills. Depression can also play a role. Avoid medications that are apt to make your symptoms worse: sedatives like xanax (alprazolam) or opiates like vicodin. Learn mindfulness (see ucla site).