On the work area you should see a InDesign Tool palette, most likely in the upper left-hand corner. See the button with the "T"? This is the Text tool that you will use to add text to your poster. Click on the Text tool and move it to wherever you want your text to be; then click.

Note: Notice the buttons on the tool pallet with a small triangle in the lower right corner. This mark indicates more tools for the button. Just like in Illustrator and Photoshop, you access these by clicking the left mouse button and holding it down for a few seconds until the nested menu appears.

To add text, choose the type tool and click the screen where you want the text to go and drag to create a text box.

Now click inside the box to begin typing.

This next part is nifty. Pay attention:

See the handles on the text box that is larger than the other handles. One is on the upper left and another is in the lower right corner... If you were to type more text than would fit in the text box a red plus sign would appear inside the box in the lower right, indicating the text is being cut off.

You can either make the text box bigger or you can click on the red plus sign and then click on your layout to continue the text within another text box.

If you click on the upper left handle you will add a new text box before the existing one. If you click on the lower right handle you will add a new text box after the existing one.

Try it.