The first time you launch Lightroom CC it should find your previous catalog (which contains your work) and convert it to Lightroom CC. This should happen automatically, I think you just have to click on OK when it asks you if you want to upgrade your catalog.
DJ has you setup for updating your catalog. (The pictures stay where they are on your hard drive(s).)
Transferring your presets may depend on a few things.
If LR3 does not store your presets in a version-neutral location, you will need to copy them from the LR3 preset directories to the LR6 preset directories.