Any cheap multi meter (less than £10 from Ebay,) will be adequate for testing voltage and continuity on a car, but don't be tempted to use it for serious electrical work at higher voltages despite what the vendor may claim.
However you will need to learn how and where to use it to avoid injury to yourself or damage to the car. That's beyond the scope of what we can do in a forum like this, so you may be better off employing somebody who knows his way around a car.