To measure analog voltages, a voltage divider with resistors is used.
Keep the total impedance of the circuit below 10k, or the analog values might be less accurate.

For example a resistor of 22k from 12V to Arduino analog input A0. And a resistor of 10k from A0 to GND.
The 22k and 10k resistors make a voltage divider. With those values you can read up to 16V.

I am fairly new to using an Arduino. I was unable find where you can post a new topic so I am posting here looking for some help (sorry).

I have just built an rpm tachometer using an Arduino and producing the results with an LCD screen. I was just wondering if there is any chance that this data could be used with real time data using say for instance MATLAB?? Any sort of help possible would be greatly appreciated!