Where to Focus Pt. 1
Some images have a definitive subject like a person or product so that's what you need to emphasise with careful focusing. With images like landscapes the whole scene might be the subject so you want to emphasis all of it - we'll look at that in where do I focus part 2.
You need to think about the image you're taking too. For example focusing a portrait which is best with a soft background (shallow depth of field) needs a different approach to focusing a landscape where you want front to back image sharpness.
With an image like this one there are three options. A shallow depth of field focused on Lawrence makes him sharp and the subject of the image because the skateboard is a bit soft but still recognisable.
If the skateboard is the subject such as in a commercial shoot for the manufacturer, you still need a shallow DOF but this time focus on the skateboard and make Lawrence a bit soft. Or maybe you want both as sharp as possible? Now you need to use a small aperture for max DOF and focus just behind the skateboard.
Focal length plays a part in this because it has an impact on DOF. If you're using a wide (short) lens there's more front to back sharpness. Check out the video because I'm running out of space here now...