"What's up," "Cool" and "I'm beat" are among commonly used American slang words. Like many slang words, the key to understanding their meaning is to avoid taking them literally.
The phrase "What's up?" is used to inquire how things are going with the person being greeted. A "cool" person is one who is sharp, in the know and so in control of his life that others aspire to be like him. "I'm beat" means that the speaker is tired, usually after having worked very hard.
Another popular slang term is "hang out," which means to go out together or otherwise spend time together. This term is usually used with friends or friend groups, rather than romantic partners. "Chill out" means to relax or to calm down. To be "amped" means to be excited, and to "have a blast" means to immensely enjoy an event or situation.
To be a "looker" means to be an attractive person. A "rip off" is something that cheats or otherwise takes advantage of person, but someone who is "ripped" has very well-developed muscles. A "sick" event means that it is cool, exciting or awesome. A "loser" is an undesirable person, and a "geek" is a person who spends most of his time studying or focused on non-social pursuits.