Just depends on who you’re contacting. Here’s how:

To someone who's not on your Messenger List yet:

• Click the Search button just above the Messenger List (it looks like a magnifying glass) to reveal the contact action bar.
• Type the person's Yahoo! ID in the box that appears.
• Press Enter (on your keyboard) or double-click the person's name in your Messenger List. A conversation window opens.
• Type the first thing you want to say, and press Enter on your keyboard. That’s it—This thing is happening!

To a contact who's already part of your Messenger List:

• Double-click the contact name.
• Type your message and press Enter on your keyboard.
• Umm - that's it! :o)

To quickly find one of your contacts who you’d like to IM:

• Click the Search button just above the Messenger List (it looks like a magnifying glass) to reveal the contact action bar - or just start typing to reveal it automatically. Type their name or Yahoo! ID in the box that appears.
• You’ll see any contacts whose ID matches what you’ve typed.
• Click or use the up and down arrow keys to select the right person.
• Press Enter or double-click the contact's name to open a conversation window and start your message...

Note: You can always send an IM to a person who is offline. The next time they sign into Yahoo! Messenger, your message will pop up for them automatically.