Each class progresses your BAB by a certain fraction; .5 for classes like Wizard, .75 for ones like Rogue, and 1.0 for ones like Fighter. At each level, add that number to your BAB total, and for the purposes of attack round down to the next whole number.

For example, a Cleric 3 Wizard 2 Fighter 3 would have 2.25 + 1 + 3 = 6.25 BAB.