how to clean saucepan?

Assuming there's nothing burned on, just use a soft scrubber (i.e., dobie pad, Scotchbrite, etc.), dishwashing liquid, and hot water. If you're using stainless, and it seems to be stained, go over it with Bar Keepers Friend.

BKF comes in both powder and liquid forms. The liquid is about the next best thing to useless, IMO. So go with the powder, forming it into a paste as you scrub with it.

If carbon steel, skip the soap, and treat it like cast iron---hot running water and a scrub brush used for nothing else.

If you've burnt something in the saucepan (or any pan) take some "washing soda" (arm and hammer makes it in the states) - a couple of tbsps - and fill it with water. Bring to a boil and then shut the gas and let it sit overnight. You'll find the burnt stuff just lifts off. If you can;t find washing soda, you can try baking soda, but it's not nearly as effective and you'll need more.
I tried it on aluminum though, and it did something weird. (Chemists, you can give the explanation - made a patina). It's a great method for stainless though.