To add an item that will have a tooltip to a chat line:

1. Begin your chat (press Enter).
2. Type the type of chat you want to do (/s for say, /g for guild, /p for party, /2 for trade channel, etc. plus space).
3. Shift-click the item in your bags, mail, or elsewhere in chat. If the item name doesn't appear in brackets ([]) you can't shift-click it.

You can also get tooltip links from places like Wowhead:

1. Find an item like [Night Reaver] at Wowhead and click the "Link" button (usually at the upper right.
2. Copy the line the popup window provides which will look like: /script DEFAULT_CHAT_FRAME:AddMessage("Shift-click this to place a link into a chat message: \124cff0070dd\124Hitem:1318:0:0:0:0:0:0:0:0\124h[Night Reaver]\124h\124r");
3. Switch back to World of Warcraft and paste the code into your chat window and press enter.
4. You should be able to shift-click the link that appears into other chat windows.