The Bronsted Lowry definition of an acid and a base is that an acid donates hydrogen ions, whereas a base receives the hydrogen ions. The Kb is the base dissociation constant, or the way in which the ions that compose the base separate into their positive and negative components. The Ka is the acid dissociation constant. The larger the value of Kb, the stronger the base, and the larger the value of Ka, the stronger the acid. By multiplying Ka by Kb, you receive the Kw, or the dissociation constant for water, which is 1.0 x 10^-14. When finding the Kb from the Ka, it is necessary to connect these various parts of the equation.
Read the problem, and write down the information that is given. In a problem that involves calculating the Kb from the Ka, you are usually given the Ka and the Kw. For example, you may be asked to calculate the Kb of the chloride ion. The given Ka of the conjugate acid of the chloride ion, which is hydrogen chloride, is 1.0 x 10^6. The given Kw is 1.0 x 10^-14.
Write down the equation for the the Ka, the Kb, and the Kw, which is Kw = (Ka)(Kb). Solve the equation for Kb by dividing the Kw by the Ka. You then obtain the equation Kb = Kw / Ka.
Put the values from the problem into the equation. For example, for the chloride ion, Kb = 1.0 x 10^-14 / 1.0 x 10^6. The Kb is 1.0x10^-20.
TL;DR (Too Long; Didn't Read)
Online, or in a chemistry book, you can find a table of acid and base dissociation constants.