Use something which does not smear. Use something which is comfortable in your hand. Try a medium point pen, though if you write small use a fine point, and if you write large you can get away with a broad point pen. Always have lots of the right kinds of pens. Most debaters flow on yellow legal pads.
Some debaters buy a ream of white legal size paper and just use that as it is more economical.
Legal paper in pads allows you to have several pages attached together at the top. There are 8 speeches in the debate, but you will only need 7 columns. This is because the 2NC-1NR occur one right after the other without an intervening affirmative speech, so they can share the same column. My advice is to draw these columns in on your pages well before the debate starts. You should flow the entire debate, even after you have given your rebuttal, so that you can help your partner.