How do I detect viruses, worms, malware on my mac?

I know that there is a way to "help" a virus or malware to take some control over your macintosh machine by asking you to allow it to make changes in your system. Now, I want to know, how do I know that I have any malware or viruses? And if I detect them, can I just delete the item like normal in the trashfolder, or do I need to have a program for removal?