One of the most important factors in your site's speed is image size. Slow images can slow your site down, and they really won't make your site look any better.
In this video, you'll learn how to optimize the file size of your images using WordPress only (avoiding external software such as Photoshop).
There are two plugins recommended in this video:
If you want to learn the basics of WordPress, watch the (100% free!) WordPress Beginner class on YouTube.