Cleopatra VII is the famous Cleopatra, the one who was involved with Julius Caesar and with Marc Antony. She died in 30 BCE. Because she died so long ago, and in a society without newspapers and other such reliable sources of information, we are not completely sure how she died.
The most common belief is that Cleopatra killed herself with snake venom. Ancient Roman sources allege that she killed herself by causing a poisonous snake or snakes to bite her. There is no proof of this, but neither is there any proof of other theories, such as the one that argues that she was murdered by poison.
So, we cannot know for sure how Cleopatra VII died, but the most common story is that she committed suicide by poisoning herself in some way.