If you're laying out a brochure or something with a lot of text, then you'll want to use InDesign because of all the formatting options.
If you're adding a small amount of text or headline or something to a graphic, it can be done easily in Photoshop.
In your tool bar you'll see an icon with a letter "T" on it. That's your type tool. When you click that you have two options with it. You can either:
1) Single click anywhere in your document which will bring up the type cursor where you can just start typing.
2) Click and drag which you'll notice will create a text box for you to type in. The size of the text box can be altered by using the handles in the corners/sides of the box.
Hope that helps.
"The game is to be sold, not to be told"