It's generally best to descent in a slightly nose-down attitude, or on a trainer, level. A nose-up descent is possible, but you're flying pretty close to stall at that point.

You shouldn't push the nose down and dive at the ground though. That just builds up airspeed, and the result will be a balloon climb when you pull the stick back to flair.

It's normal to need to hold a little back stick while descending at low power, even though the nose is level or slightly down.

When flairing, you only pull back as hard as you need to prevent the nosewheel from touching first, or on a tail dragger, to prevent touching down in a nose-down attitude (or too nose down), since that often results in a nose-over.