Add new slides for content. Press Ctrl + M for a new slide. A blank slide will be added after the slide you are currently on. The slide will contain a title box and a text box. You can choose to use these or insert your own objects using the Insert tab.
When adding a text box, you can click and drag to make it whatever size you'd like. You can then adjust this later by grabbing one of the corners with your cursor and then clicking and dragging again.
You can click on any text box and start typing to begin adding text to your presentation. You can format text just as you would in Word, with formatting options available in the Home tab.