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.