Dharana is the 6th step of the Ashtanga yoga and is single minded concentration on an object.
This is something you can only accomplish by regular practice. When you try it you will notice how difficult is can be. It is very important that you choose the object of you concentration carefully, as it has to be something you can concentrate on spontaneously without pressure. If you try too hard to concentrate you will end up grinding your teeth and tense the facial muscles. Do not try to force the concentration, because as you know Dharana leads to the next step Dhyana, do not fight the mind in order to obtain your goal.
Forced concentration is not real concentration and it creates tension in the mind that distracts us from the path of Dhyana. We should confine the activities of our mind within the limited sphere of the object.
As soon as the internalization of the senses (Pratyahara) is complete, you can learn to direct your senses onto a central point of an object. Dharana will flow into Dhyana as soon as you can maintain a fluent, undisturbed stream of awareness towards and internal symbol for a certain period of time.