Tap the Annotate button on your right-side menu.

For simple text, tap the Text option (Selected in above image)

Tap on your canvas where you want it to appear.

You'll then see the text box appear and you'll be able to type. Once finished, tap Done.

Tap on the Annotate icon from the right side toolbar and then slect Text Callout option.

Press anywhere on the canvas where you want the callout to point to

Drag out and away from where want the text to appear. Then add your text and tap Done.

Use Freehand Annotation to handwrite out text on the screen.