The neck should be in a relatively neutral position. In other words if you are on your back, your head can be slightly elevated or flat, but the neck should not be too flexed. If you sleep on your side, the pillow should take up the space from the mattress to your ear so that the spine is aligned from upper back to neck. Sleeping on your stomach is the worst sleeping position because it forces you to rotate your head for long periods of time.
When should you buy a new pillow? About every 18 months. Memory foam pillows typically last longer, up to three years. If you’re using a pillow that’s five or six years old, you’re not getting the support you need—and you’re not sleeping as comfortably as you could.
The firmness of the pillow is totally an individual choice. But keep in mind that a soft pillow, like down, might provide support for a few hours then flatten overnight, leaving your head and neck in an awkward position. An orthopedic pillow, which has a space for your head is great...but only if you find it comfortable, which many people don't. If you purchase an expensive pillow make sure there is a trial period so you can return it for full refund if it doesn't work for you.
If you are a light sleeper and toss and turn, you may want to have several pillows which you can switch out during the night, for example a thicker pillow for when you are on your side and a flatter pillow for when you are on your back.
A word about the "My Pillow" infomercials - I bought two (insert sad emoji). They are stuffed with one inch size pieces of irregular shredded foam. Very lumpy. Not good. Look elsewhere.