I think just about every microcontroller in the world must have a UART compatible with RS232; however, RS232 specifies the use of bipolar signal levers (about +/-12V) - and I don't know of any microcontroller which does that directly.

You need to add a component called a Transceiver that converts between the 0-5V (or 0-3V, or whatever) of the microcontroller and the +/-12V of RS232. The MAX232 is the classic example, although there are plenty of variations on the theme!

The Transceiver also does other signal-conditioning required by the RS232 standard, and provides protection against static discharge and other abuse that happens when you start plugging-in external cables...

If - as I've already suggested - you buy yourself a ready-made kit, this will all be included for you.