To help explain multicalssing, let me assume that the example you are using is a fighter with a BAB of +14/+9/+4 and you are adding a level of rogue. All sources of BAB stack, so you add the rogue's 1st level BAB of +0 making his new BAB +14/+9/+4. Now you add a second level of rogue. At second level, a rogue gets +1 BAB, changing the total to +15/+10/+5. You can only add one level of a class at a time, you do not add the equivilent levels of your new class when you decide to multiclass. In the ezample above, you started with a 14th level fighter and when he accumulated enough points to become 15th level, you chose a new class to advance in instead of the same class making him a 14th level fighter/1st level rogue. This continues until either you add a third class (starting the count for the new class at 1st level as well) or you take another level in your first class, raising it instead. You can swap back and forth at will each time you gain enough experience points to advance to a new character level (the character level is the same as the total of all your levels added together). There are a couple of classes that impose restrictions on multiclassing, so refer to the PHB before you make that decision. This same theory is applied if you take a level in a prestige class. They just have their own entrance requirements that can be referenced in the DMG.
