Transistor with arduino

hey guys, have been using transistors arduino test them out, have few questions them.

first of all, don't know resistor use  @ base, have been using 3.3k , 1k ohm resistors, don't know if turns on transistor. kind of formula/equation how current need apple apply base of transistor , resistor use.

if explain me little more part of transistors , equation used, great

by way, using 2n3904 npn transistor farchield semiconductors
data sheet:
http://www.fairchildsemi.com/ds/2n/2n3904.pdf

thanks much

i write tutorial in french in forum french section :
http://forum.arduino.cc/index.php?topic=100727.0
if don't read french can read graphics , formulas 

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