I am planning to build some H bridges with relays to control 4 car window motors. I think that they need around 20 amps each motor. Right now i dont have any idea to how to build the circuit so it can be PWM controlled by an arduino. Any ideas?

Edit. I forgot to mention that i dont want to buy already builded parts.

 

The controller does not provide the amps, it provides the control signal.

The drivers, that drive the motor, provide the amps.

 

Unless you are doing it for the exercise why not look at picking up a couple of drives from the auto wrecker, I believe they are PWM, but they are fed from the Canbus network usually and come with electronic breaker/ O/L, but may be possible to reverse engineer to interface to Arduino?
There are many circuits out there for PWM controlled DC brushed motors
Max.

 

The VNH2SP30-E by ST Microelectronics is just the ticket for what you're wanting:
http://www.st.com/web/catalog/sense_power/FM1965/SC1039/PF145473

Mouser stocks them:
http://www.mouser.com/ProductDetail/STMicroelectronics/VNH2SP30-E/?qs=%2bX2uje6VeKVvuwsn%2b4MAoA==

Unless you're really determined to make your own board, you could just get one of these:
http://www.pololu.com/product/706

If your goals are to save time and money and have a reliable driver when done, then definitely go with the finished board.

If you're determined to build your PWM bridges from discrete components, you're likely going to spend a good bit more money, a LOT more time, and have a number of smokey deaths of boards, then go ahead and build one yourself. I suggest very strongly that you read through ALL of the documentation on those VNH series of controllers, as you may want many of those features to be in your finished project.