Appreciate your asking for me but anyone can help you.
I guess the very first thing is to discuss this where you just bought it as it's going to need something.
Is this a dealer sale? Any warranty? Please tell us what year and type of vehicle it is.
There are many reasons it could overheat, some simple and some not. We'll need to know more. Do check and fill coolant for now and don't allow this to overheat again. Yes - it's a bummer just 20 miles into a new car to you but let's not allow whatever is needed to make things worse.
Talk to seller at once. See if you can see something obvious like a hose split open or a place that leaks out just from refilling it with proper coolant - usually 50/50 anti-freeze and water.
Tell us more when you can,
T
Tom Greenleaf - MetroWest Boston - USA
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http://www.autoacsystems.com/tomgreenleaf/