How do I know when to start a new paragraph?
A paragraph is a block of sentences that communicate a solid idea. You begin a new paragraph when the idea you want to communicate is fully explained and you're ready to transition into another. It is a package of information. Let's try this visually: draw a box on a piece of paper. This box will serve as your paragraph. The box holds different ideas. Let's package your main idea into the box and circle it to show it is the main idea. You may now write-in several supporting ideas (like your examples, quotes, and research) around the circled main idea. There you have it. For transitioning to the next paragraph, you might try drawing a line or wire from one of your supporting sentences into another box with another main idea of similar nature. You could even outline your whole paper this way as represented by a stack of boxes packaging the information you want to send to readers.
I'm trying to use up as much white space as I can for this composition, but I'm not what else will fit with it.
I understand. I wouldn't worry about trying to occupy as much white space (or rather negative space), in the composition. If a composition is too colorful or complex, it will be easy to overwhelm the eye. Good composition works best when it is simple but can still communicate an idea effectively. As long as the piece can allow your audience to look at, absorb, encode, and understand, it is effective in the practice of graphic design.
I know we never end a sentence with a preposition, so how would I properly end a sentence such as "Where do you want to get to?"
Good question. Think about how you can rearrange the parts of the sentence so that the "to" isn't at the end. Alternatively--perhaps even better yet--think about what word or words would say the same thing. Let's try this: circle the three words, "to get to," at the end of that sentence. Think of a word in the English vocabulary that will sum up that idea. For example, "go." So your sentence will say instead, "Where do you want to go?" In English (be it speaking or writing) less is more. The more you can say in fewer words, the better. It's actually kind of fun to practice.