It's pretty simple, actually. The way you'd do it in any other program. Simply find the text tool (It'll show a capital T). If you can't find any tools at all, go to the upper menu of Photoshop, go to Window, and then click Tools to let it appear if it isn't already there.
In your working area that should exist, simple click and drag to form a box in which you will be able to type. If the font is large and the box is small, you may not be able to see any text. If this happens, simple increase the size of the box by dragging its edges or decrease the size of the text by selecting it (Ctrl + A to select all) and then near the top, changing the numerical to a lower one.