Plus of led strip to 12V, minus of led strip to collector of TIP120. Emitter of TIP120 to ground. Base of TIP120 via a resistor to pin of Arduino.
You will need to calculate the resistor; a guide can be that the current through the resistor needs to be approximately 10% of the current that the strip draws. The current through the resistor may not exceed 20mA (the safe current that a pin of the microcontroller can deliver).
Note that transistors in this application can run very hot; it's better to use logic level fets. In that case you do not need to calculate the resistor, just use the value that was shown in e.g. the article that I linked.