It's pretty easy to turn any of the text in your email into a link. First, highlight the text you'd like to turn into a link. Once you do that, click on the link icon in the text toolbar:
You can then choose from several types of links to add to your highlighted text. Standard Page lets you link to any page on your website that is not part of a store or blog. Product and Category Page let you choose a specific product or category from your store, and Blog Page allows you to choose a blog post to link to.
You can add multiple links to your email, but you shouldn't turn the entire text of your message into links. Think of links like whipped cream - pretty good on it's own, but even better when there's some pie underneath it.