I'd like to learn

[Vipershark]

image posted for reference since it's a new page

Okay, first things first, please don't ONLY post the big version of your sprites.
You should always post them at their original 1px scale (as well as the bigger versions for detail if you choose to do so) so that the sprite can be seen how it's meant to look.

Anyway, now that you've gotten rid of most of the jaggies, you're left with lines that are cleaner yet are still sort of all over the place. Notice how the top of the hat looks bumpy instead of being a smooth curve and Mario's nose looks sort of octagonal rather than round, for example. Your next step is to go in and clean up your curves and and lines to get your lineart looking good.

I'd post an edit along with this but I'm on my way to work.
I'll probably do one later tonight.

edit- Make sure you look at a good reference when cleaning your lines.