The most asked question I receive is which pen do I use. I sketch mostly with
waterproof fineliners.
One of my favorite fineliners is
Sakura's Pigma Micron, but there are many options in the market.
You could try as well fineliners from
Staedtler,
Uni-Pinor
Faber-Castell.
The most important is that you choose a water resistant one, so after you apply watercolors, the ink won't bleed.
The brands I mentioned above offer pack of fineliners with different tip sizes.
For beginners I always recommend choosing a pack with few different tips.
Only by trying you can find out which one you like most.
I usually sketch with mid-size-tip like 0,4 or 0,5mm.
When the sketch is smaller or when I want to make it look lighter, I use 0,1 or 0,2mm tip. For adding extra contrast, I use 0,8mm tip.