How to achieve shallow depth of field
The best way to achieve a shallow depth of field is to use a Tele lens or a relatively long focal length. secondly fill up the subject as much as you can, i.e reduce the distance between the lens and the subject or in other words be somewhere close to the minimum focusing distance of your lens. Further, keep the aperture as open as possible keeping in mind the aperture-shutter speed exposure combination. Keeping the aperture wide open may sometimes lead to too shallow a depth of field that you may not get all portions of your subject covered within the shallow depth of field. This may be desired at times, which is why it important to always focus on the eyes as the eyes are are the most important area in a photograph and we can't afford to have them not in focus. To counter this, you may arrange your subject in a manner so that there is maximum possible difference between the subject and the background. Now you may close down the aperture a little bit more as the increase in depth of field will only influence the subject and not the background.
For a shallow depth of field
1. Open up the aperture as much as you can.
2. Shoot with a longer focal length lens
3. Be closer to the subject.
4. Let there be a lot of gap between the subject and the background.
5, using a prime lens like 85mm or 135mm is also a good idea as you can physically focus at a closer distance with these lenses.