converting to fixed (new)
 

okay im pretty new to fixed gears.

but i got my friends old road bike, and i was wondering how would i know if i have to change my bottom bracket?

Unless you ghettofix the old hub and respace/redish it you'll have to change the bb if the chainline is way off from 42mm(standard chainline on fixed hubs).Check out sheldonbrown.com for info on conversions.

alright thanks

Standard chain line for road double cranks is 41mm for the inner chainring and 46mm for the outer ring. On some inner rings, the teeth are offset to the inside from the plane of the chainring. Turning it around so it offsets to the outside will bring the teeth closer to 42mm.