In order to remember a dream, you have to wake up at some point during the dream or at least be in the process of waking up. If you know you dreamed, but forget what the dream was about, that is normal. We begin to forget details of a dream within about 5 minutes of waking. If you want to remember you dreams in detail, keep paper and pencil by your bed and write what you remember down as soon as you wake up. Good luck
You will forget the dreams that aren't very vivid. And depending on when you were in this dream world etc. If you are just waking up from a dream wait in bed for a little bit, lay back on your pillow and try to recall the state of mind you were in carefully and you may remember your dream. If you want to badly enough recall your dreams write them down. Something I dont ever do but, i hear it works.