Here's how to add them:
1. From your Blogger Dashboard, click Design.
2. Click on Template Designer from the upper left hand corner.
3. Click on Advanced from the upper left hand corner.
4. In the 2nd column from the left (next to Advanced) scroll down until you see "text font". To change the text font, click on text font and then scroll through the list of fonts. Click on the fonts to view them on your blog.
5. Make the text Bold or Italics by clicking either B or I.
6. To change the size of the text, either use the drop-down menu or enter a value (a percentage) yourself. If you use the drop-down menu, all of the text will be the exact same size. This means that your post titles, sidebar text, post text, etc. will all be the same size. Try it out, often this will look funny. If you enter a percentage, then the text will increase or decrease in size but they will do so proportionally. That means, if your post titles were twice as big as your text, then when you change the percentage to 120%, the post title will still be twice as big as your text, but they will both be 20% bigger than they were before. Enter different percentages and see what you like best. Lower than 100% makes the text smaller, higher makes the text bigger. (Click outside of the box, in the gray area, to see the changes applied.)
7. Repeat this for the Sidebar Title Font (which includes the date), Blog Title Font, Blog Description Font, and Post Footer Font (the text where your labels and comments are).
8. Once you like what you see, click Apply to Blog in the top right corner and then either View Blog or Back to Blogger.
9. To go back to your blog without saving changes, click Back to Blogger. Your blog will look the same as it did before you went to the Template Designer.
Here's an image of what you will see when you edit the fonts (click on the image to see it bigger):