What's Up?  When friends talk together, we often ask, "what's up?"  The answer is often something like "not much" or "I'm just watching TV".  It basically means, "What are you doing now?"  We NEVER say "How are you doing?"  It's really too formal and weird!

Up To:  We often ask a friend, What are you "up to" later?  This "up to" means "doing".  It's the same as saying, "What are your plans later?"  Asking "What are you up to?" is much more common and a more natural question.

Staying in:  In this context, native speakers say "Staying in" instead of saying "Staying at home".  It's much more natural.  If you hear that someone is "staying in" it means they are staying at home and not leaving the house.

Chilling:  This is a very common slang word that we often pronounce as "Chillin".  It means something like "Relaxing" "watching DVD's" or something like that. 

Swing by:  If you "swing by" someone's house it means you "go there".  We use this phrase a lot.

I might swing by the bar after work and meet you guys for a couple of drinks.  It will depend on when my meeting finishes and how tired I am.

Cooped up:  If you are feeling "cooped up" it means that you are feeling like you have been staying inside the house or the office for much too long.  This is the feeling that makes you want to leave the house and go outside.  A common example sentence is:

I've been cooped up in bed all week with this stupid flu.  I'm better now and I need to get out of the house.
Let loose:  If you "let loose" it means that you allow yourself to have fun.  Maybe after a long work week or a long school week, you need to see your friends and just have fun.  This is called "letting loose".  It means you forget about your everyday stress and problems and just have fun.

Be up for:  If you are "up for" something it means you are interested in doing it.  Here's a perfect example:

I'm really too tired to leave the house today but I'd be up for having lunch together tomorrow.

I'm pretty beat:  If you are "beat" it means that you are really tired.  Here is a sentence.

I'm so beat from work lately.  I really need to take a huge nap.

Get a second wind:  If you "get a second wind" it means that you were tired before but all of a sudden you got some energy to do something.  Sometimes you might be really tired at 7:00 pm but later you might get a "second wind" at 9:00pm and be ready to meet your friends.

Later:  We often end phone conversations with "later".  It means the same thing as "bye" but it is more common between friends.