crystal osc.

Study the uP application notes...they will have an oscillator section, trust me.
Google crystal oscillator to find out how it works.... not too hard to do.

Or crystal oscillator circuits...

Del

As Del says the data sheet will have a section on the XTAL inputs. This is part of the MC68HC11 data sheet with my additions in blue:-

Essentially the two pins X1 and X2 have an inverter between them: this enables you to either add a crystal and associated components to generate the internal clock for the IC, or, to input a square wave at the required frequency at pin X1 (EXTAL). This circuit shows you how to use one crystal to generate the clocks for two µprocessors, and so, shows both methods in one diagram.

Just connecting one side of a crystal to the X1 input won't work at all.