By this point in your lives, I anticipate you will be fluent with chat and IM clients.  So what you do with them is not the issue here; how you use them in-world will be new.

The essential difference in SL is that chat is real-time communication for avatars very close to you. They all will hear it and if you have your logging feature enabled, you'll have a written record of what's said.

Instant Message may be used for privacy with one person near you (just right-click their avatar and choose the option to send an IM).  Here Iggy sends Jezz an IM as she camps, earning a few Linden Dollars by mopping the floor in a desolate shopping mall:

(I have NO idea what that pink-headed thing is beside her...seems to be another avatar stuck halfway through the floor).

IM is also popularly used to contact someone anywhere else in SL, even offline (if you have the "Send IM to e-mail" option checked in under Edit Menu --> Preferences --> Communications tab.

Chat is easy, and Jezz Enoch will demonstrate for the class.  She's trying to hire a bad guy to kill her brother, Pappy. She picks a Welcome Area that is full of monsters and other riff-raff.

To get this bad-boy avatar to go after Pappy, Jezz walked up close to him and types her chat message into the chat box at the bottom of her screen:

To make it appear in-world so her new friend can hear it, she hits the enter key and this appears:

After sealing the deal, Jezz sends Pappy an IM.  He's not online at the time (being dead drunk) but Jezz wants to taunt her brother. She finds his profile with her friends list and clicks the "instant message" tab on Pappy's profile:

When Jezz clicks this...she opens a box so she can send a message to her beloved brother. She types each line and then hits enter to send it. Note that the "user not online" message just shows that Pappy is indeed drunk, and not hiding somewhere on Richmond Island.

Scavenger Hunt Reward: 50L to the first person to send Pappy Enoch an Instant Message (fluency in Hillbilly not required).