Turn on the text tool (looks like a letter A), click somewhere in your image, then start typing. For the sake of being easy, after done typing, click on a a different tool (any other tool is fine), then click back on the text tool. Click in your document and type again. You now have 2 different "text boxes" using the arrow (select pointer tool) click on either of the text boxes and you can format the text differently.
Hope this helps.
Just so you know though, you can have multiple fonts within the same text box. You just have to highlight the text with the text tool and using the property panel specify the formatting.