AVR. How to configure as output just one pin?
PB5 is the 6th bit.
Just remember that the numbering is zero-based and starts with the least-significant-bit on the right, so the value you're looking for in this case is 0b00100000.
When you're setting this bit, you'll probably want to use a logical OR operation instead of a simple assignment, so:  

DDRB |= 0b00100000;  


instead of

DDRB = 0b00100000;  


That way you won't inadvertently clear any of the other bits which might already be set.