Cyangmou, you inspired me to buy some pencils and get to it (30 bucks CAN, owch).
I recommend col-erase for the under sketch. ------> can also be used for the over sketch.
Non-Photo Blue is really nice and plasticy, doesn't smudge, and works really well with a light table if you have one -----> but isn't necessary.
It's what most paper animators use, but it's also good for just plain sketching.
Easy to draw over, visually recedes, adds a bit of character to the drawing, and if you really care can easily be removed ---> hence the Non-Photo.
Here's a place to get them.
http://www.dickblick.com/products/prismacolor-col-erase-pencils/Look around for other places to buy, better prices than the one I linked probly exist.
I think I payed around 20-40 cents a pencil but can't remember where.
Gray is also good, can be light or dark depending on how much you apply, mostly smudge free -----> a lot less than graphite.
The erasers on the back of the pencil are not great.
Get a hand ersaser.
Here's a few I like.
Magic RubSakura SJ-100Tri TipSome old examples.
NP-Blue undersketches:
1 2 3Gray and Various col-erase colors:
1 2 3 4If you're just starting out no need to go crazy.
I'd say get like 3-5 NP-Blue and 3-5 gray to see if you like them.
From there try a few other colors.
Be warned, the other colors vary in consistency a lot.
The NP-Blue/Pink are very hard like plastic and don't smear.
Scarlet Red is moderately hard and smears a bit.
Indigo Blue/Brown/Terra Cotta are soft and smear a lot.