HI guys, I am not very experienced with Arduino and for my project I want to toggle the fan connected to an external battery on/off using sketch instead of a normal switch. Any advice on how to do it would be great.
Thanks for your time. 
Welcome to the forum.
This is a very basic use that has been covered many times. A simple search in the box above or reading the Learning tab contents will answer this question.
Look for "using a switch" and "turning on a motor".
We will be happy to help with any problems but you have to do some basic research first.
Weedpharma
My problem is i don't want to use resistors because it works perfectly with just battery.
I just need to control the motor with arduino. All the projects in the libraries and on the net have resistors but i don't. That's why i said to control the switch using codes.
Thanks anyway.
Edryu
You give insufficient information to give a precise answer to your problem. I will take a guess at what you want to do.
Use the Arduino output fed via a R of around 470 ohms to the gate of a logic level MOSFET. Use this to switch the motor on. Don't forget to put a reverse diode across the motor to prevent reverse voltage spikes.
You could also use a relay shield to drive the motor.
You do not even say if you are having trouble with the hardware or with the program.
Weedpharma
According to that tutorial , motor should be easily controllable bi-directionally , I want to create the same setup . How does it work on the software side because that tutorial doesn't work for my motors (two , both use less than 3 V.) That's all I have to say..
thanks,
Edryu
Please supply details on the motor as well as a circuit of how it is connected and powered.
Please put your code in its own window as seen in other posts. This can be done by placing   [code] and [/code] around the code or use the </> icon. This makes it easier for others to read.
Weedpharma