Nightmares have many causes: sleep deprivation, midnight eating, psychological reasons, recent life changes,stress or anxiety, horror movies, etc. Nightmares can be ceased when you reach the root of the problem, look around at your current atmosphere and see what may be causing the nightmares. We all have weird dreams from time to time(although our perception of weird differs from person to person), it is not uncommon and neither are nightmares. As for dreams coming true, there are cases when one forsees specific events through dreams; it can happen and has happened to many.
dreams that come true are called prophetic dreams, i've had a few, but i can't help you with those because i'm not 100% how they work. i can just tell which dreams are prophetic or not by various symbols and feelings. some theorize these dreams are the result of your subconscious picking up on subtle clues in your waking life over a situation.
nightmares are commonplace, same with weird dreams. it's a combination of waking fears and your mind working with your imagination and symbolism to create a story. now, if your nightmares are reoccurring over a short period of them [say 3 times a week] or you start experiencing night terrors or trouble sleeping, you should speak with your doctor since it could effect your health.