My project is to ramp a voltage between 0-6V
The arduino will only provide a 5V output. If you do need 6V then you need to pass the output through a transistor connected to a 6V power source.
The arduino provides a PWM output, you can smooth this to get an analogue voltage.
If you don't know about PWM then see:-
http://www.thebox.myzen.co.uk/Tutorial/PWM.html