While it is possible to use an SCR to control a DC motor it is not a good way to do it, as it causes distortion of the mains and may allow DC to flow in the supply transformers. Generally the motor is driven by pulsating DC that is derived from the mains (transformer), and the turn on time of half cycles is controlled to vary the width.The control is not very linear. An SCR control is more likely to be found in older equipment or larger industrial machines. It cannot respond quickly, because each cycle has to be completed before the next can be controlled. There are complications caused by this, leading to erratic control, especially with smaller motors and feedback control of speed.
It is best to look into using transistor or mosfet devices as switches. These can give smooth control of speed, and with a H bridge circuit reversing is possible. These are especially suited to low voltage DC motors. This is what is found in a battery operated drill for example.
The first link shows a practical H bridge. The speed can be controlled by driving this H bridge from a pulse width modulated source, which can be as simple as a pair of 555 timers as an oscillator with a knob for width (PWM). PWM can also be generated by dedicated ICs, or a microcontroller. The frequency may be from a few hundred Hz to thousands of Hz, so the response time is quicker, and modulation of width is smooth. The faster pulses are also more easily smoothed by the motor inertia.