You can create a new font like this:
Font verdanaFont = new Font("Verdana", 20f, FontStyle.Regular);

You can change Verdana to anything you want: Arial, Arial Narrow, Comic Sans MS ...
Change the number before f to change the size.
Change font style to anything you want: Bold, Italic, Underline ...

For setting default font just write the code after InitializeComponent. For example:

using System.Drawing;

public Form1()
{
InitializeComponent();
Font verdanaFont = new Font("Verdana", 20f, System.Drawing.FontStyle.Regular);
richTextBox1.SelectionFont = verdanaFont;

}

So when you start typing the font will be Verdana.

Umm.... richtextbox.FontFamily?

Tried that, and various other things with a FontFamily member (like the richtextbox's Document). Doesn't have any effect.

I'm pretty sure I've tried everything obvious, but feel free to talk to me like I'm a five year old, because I feel like one right now.

I am not much into WPF applications but this code should work:
richTextBox1.FontFamily = new FontFamily("Verdana");

Just debug your program using each of these:
richTextBox1.FontFamily = new FontFamily("Verdana");
richTextBox1.FontFamily = new FontFamily("Arial Narrow");
richTextBox1.FontFamily = new FontFamily("Comic Sans MS");

You will see the difference.

Are you loading RTF into the box after attempting to set the font? According to msdn, setting that property (whether it be via XML, direct in code, or by whatever other method) will change the whole document, as well as changing the current font.