hmm, i don't think that this is a very needed tutorial, for it's really about the very
basics of linearting on a vector program. therefore i tried to make it such simple that even a ten year old boy with very poor English can understand. cuz otherwise it would really be useless. after ~wang-xiangwei
asked for a way to draw a single line with varying thickness, i noticed that there is no such option. Well, whatever, it's a very simple program indeed. and after all, it's all free.
yup. you can download Inkscape at [link]
you can import images in inkscape, so you can open your sketch in it and then lineart over it and then delete it and go with your lineart. and, of course, you can export .png image of your lineart and colour it in PS, which is the best part anyway.
by the way, if you want to draw lines with varying thickness or any such style, you better learn how to use the pain
tool of PS. good news: using Inkscape for a while will help you familiarise with nodes and handles, which will help a lot in learning the use of pen tool. i only recently discovered the options of that tool. well, it's still a pain tool, but sometimes it's worth the pain, isn't it?