First, consider how the elements of your site are arranged. The most attractive way to align items is on a grid, so check to see if your content is all lined up. If you have several pictures in a row (or one above the other), make sure the edges line up. Having some stick out farther than others looks unprofessional. Avoid centering paragraphs of text (titles can be centered, on occasion) - choose to align them to the right or (most likely) the left.
Don't feel as though you need to fill every inch of your pages with content. The empty space around or between elements, often called "white space" (regardless of whether it is actually white!), is a very useful design element too. It allows your content some breathing room and doesn't overwhelm the reader. Making use of the empty space on your pages can create a very visually commanding look. Read more about white space in web design.
And finally, keep your mobile users in mind. More and more people, especially students and young professionals, do most of their web browsing on mobile devices such as smart phones and tablets. The site editor allows you to see what the mobile version of your site looks like by clicking the smart phone icon in the upper right corner near the PUBLISH button. Check out all of your pages in this mobile viewer to make sure they show up the way you want them to, and fix them if they don't.
By employing these methods, you should be well on your way to creating a visually compelling website that will make visitors want to read your content.