There are three focusing options on pretty much all DSLRs:
1. Auto Autofocus
2. Manual Autofocus
3. Manual Focus
Number 1 is the AF default: the camera will automatically focus on what it automatically chooses to focus: push down the shutter button halfway to autofocus. Depending on your camera, it will communicate to you when your photo is in focus: usually by either flashing one of the sensors in your viewfinder, or something similar. Check your instruction manual if this is not entirely clear to you.
Number 2 is halfway: the camera will automatically focus for you, but you tell it which specific area of the photo that you want to focus on. For example, on Canon cameras with a 9-point AF system, you tap the AF selection button and then use either the thumb nubbin or the control dial to choose which AF point you'd like to use. Again, this will vary by camera: if you're not sure about yours, check your instruction manual. If there's no AF sensor near where you want the focus to be once you've composed your photo, no problem: just put one of the sensors over your subject matter, hold the button down halfway to focus on that object, and then while continuing to hold the button down halfway, recompose your photograph.
Number 3 is easy: for entirely manual focus, flip your lens' switch from "AF" (autofocus) to "MF" (manual focus). Simply turn your lens' focusing ring, and when the subject matter comes into focus in your viewfinder, you're set to go. Many lenses made now allow you to use your lens in manual focus even when switched onto AF - check your lens' instruction manual before doing this though.
Don't bother with manual focus unless you have good reason: for example, if you're in a low-light situation photographing something still (like a cityscape) and the camera is having a hard time auto-focusing. Otherwise, default to one of the autofocus settings.