Hey guys,
im trying to drive a little car using an Arduino.
There will be a 12V DC motor. So far i hooked it up with one transistor to let i turn one way.
Next step will be to switch the polarity in order to let it turn the other way around.
I was thinking i may use 4 Transistors, 2 for each direction. Id be happy if someone would confirm that my Idea works 
I can't see your picture right now, but basically yes - you need an H-bridge, either made with transistors, or made with relays (more or less limiting you to full on or full off tho).
There are several H-bridge controllers available, such as L293 or L298, do a litle searching for their equivalent if you need mroe current control (you need to take the stall current of the motors into account whens selecting your parts).
For a more detailed look at reversing a motor see:-
http://www.thebox.myzen.co.uk/Workshop/Motors_2.html
Thanks guys, yeah looks like i tried to re-inivent the H-Bridge (sorry i didn't to a deep research before posting) but nice to know i was right 
Problem solved, thanks again
CrossRoads:
I can't see your picture right now, but basically yes - you need an H-bridge, either made with transistors, or made with relays (more or less limiting you to full on or full off tho).
There are several H-bridge controllers available, such as L293 or L298, do a litle searching for their equivalent if you need mroe current control (you need to take the stall current of the motors into account whens selecting your parts).
Not necessarily - if you stuck an NPN or n-channel mosfet between the ground rail and the "low side" relays (with an appropriate current rating - and a snubber diode), you could use PWM that way; of course, going this route, you might as well make the entire thing from transistors/fets to share the load (and for really large motors, using a single device for the PWM control would probably be almost impossible anyway).
However im only going to use small motor (<1A), at first only with full speed/one speed, lateron maybe some PWM experiments