You have several options... the first of which is to read the documentation first.
Backup your site and database as is recommended before you do anything major, whichever way you go.
You can either clone your current site on a test server and upgrade to see the consequences or...
Upgrade your current site since you already have a backup and most settings are now maintained in the database.
I recommend the expert upgrade for both Joomla and CB. To my knowledge, it won't make a difference which you upgrade first, but keep a copy of your configuration files for both before the upgrade. Unlike upgrades from CB 1.0 or 1.1 and Joomla 1.0, you shouldn't have any compatibility issues of 3PD extensions with the upgrade.