How to Focus on a DSLR

As you look through the viewfinder and frame the subject, lightly press the shutter release button. With a little practice you will get the idea as to how much pressure you need to apply so as to just focus and not to actually click.

Make sure that you have selected the right focusing point which should overlapping the main subject or the area you plan to keep in focus. In case of a portrait keep the focusing point on the eyes. 

If you have two people in the frame, focus on one and then recompose the shot without lifting the finger from the shutter release button. The focus would not change and the image would be shot the way you want it.

These two people are not sharp because the focus is on the bushes behind. Check the Red centre focusing point which is overlapping the background instead of the subject. 
Although there are three people in the photography, they are all in focus because the focusing point is overlapping the subject and not the background.