The Layouts button on the left will provide you with a variety of ways to arrange your text on the currently selected slide. The options under the Layouts button will change depending on what slide type you currently have selected - if you have a paragraph slide selected, for example, what you will see under the Layouts options will be different from what shows if you have a bulleted or header/subheader slide selected.
Here's a deck I threw together to show some of the different layout styles:
Created with Haiku Deck, the free presentation app
If you decide you want to change the layout of your slide, simply select another layout. If you find that you want to add more text than any of the layouts or slide types accommodate, consider breaking your slide up into more than one slide, or putting your text in the public notes for your slide.
To see it in action, check out this video: