A few days ago I had a strange dream. I was in some kind of a cave. In the middle of the cave there was a tower of fire. And in the middle of the tower there was a pillar of light.And I heared someone saying "bewareof darkness". And then I woke up.
If someone could tell me what does all that mean, I would be greatful.

It's a bit vague and not enough information to give you an answer on what it meant. If you could provide more detail, your thoughts during the dream, perhaps what occurred previously to getting there, more about your environment and surroundings perhaps I could give you a better explanation.

It's quite possible this dream was more than just a dream and it's quite possible if it was a one minute snippet it meant nothing at all, or the thought hasn't quite formed enough yet to relay to you what your subconscious will eventually come up with in future dreams.

well now that i think about it, at first i was at a meadow. and there was someone else with me. then we entered a cave, but when i looked around there was no one else there. and then i saw an abbys (i couldn`t see the bottom). and a tower of fire was coming out. then suddenly the tower "opened" to me and there was the pillar of light. when i reached for it i heared that voice.

Forgive me, but your dream kind of reminds me of "Lord of The Rings."   Are you a fan? Did you recently read the trilogy or recently watch the movie?

In the sense of interpretation, at very best, it sounds like there is some hidden knowledge, or something that is new to you that you have discovered and are learning and your mind is warning you to tread carefully. Either because it is something you were raised to believe it is no good and frowned upon or because you are unsure of it because it is unknown and yet to be seen.

actually it has been quite a while sinse i lst saw the movie. and i just started reading the book yesterday. so i don`t think it could be related to that. and besides frodo wasn`t alone