When you first start your computer, the GRUB screen (where you choose your Operating System) appears. Select the Fedora that you want to boot into, but press the a key instead of pressing Enter.
You will see a line somewhat like the following:
kernel /vmlinuz-2.6.9-1.667 ro root=LABEL=/ acpi=on rhgb quiet
Add the number of your runlevel to the end of that line, and then press Enter. For example, to boot into text-only mode, the line would look like:
kernel /vmlinuz-2.6.9-1.667 ro root=LABEL=/ acpi=on rhgb quiet 3
You will then boot into the new runlevel this time only.
More about runlevels: http://www.fedorafaq.org/basics/#runlevel