Step 2: Calculate how much current will flow
(maybe people in England or the US know the Voltage as "V" but I will refer to it as "U" as we call it in here)
U = R * I
OK, but we want the current "I" right ? So just divide through the Resistance "R".
U = R * I / :R
I = U/R
For my relay that would be:
I = 12V / 400Ohm
I = 0.03 A => 30 mA (That is Ic)
The Arduino can handle up to 20mA but its better to use a transistor even if your current is only 20mA. So for 30mA you definitely need one.