How should I understand the amount of base current in BJT transistors?

_{(source: wikimedia.org)}

In this circuit, the base current would be approximately (Vs - 0.7) / R2. The BE junction (with the arrow) behaves like a diode, so as long as Vs is greater than 0.7 V, then current will flow through the diode and there will be a drop of about 0.7 V (depending on the part) and then the current is just limited by the resistor according to Ohm's law.