The bitcoin qt really takes a lot of memories every time when it's running. It normally takes like 3 minutes to open up!
Is there any suggestion on lighter, easier-to-use wallet?
People are saying that on-line wallet is not safe. Any suggestion on that?
It takes 3 minutes because it check the last blocks before starting
Use Multibit or Electrum, no need to have the whole blockchain.
Pure online wallet of course aren't safe, the owner can steal your bitcoin. But a online wallet like blockchain.info one is safe because they don't have access to your keys. Just make sure you make backups!
and whenever i run it, it takes up all the cpu. is this normal or happening to anyone else?
It is verifying the blocks it download, of course it use the cpu. Is it a problem that your computer is actually used? Because that is why computers exist: to be used.