Assuming you have your lineart on the machine (drawn within your favorite application, scanned, or by photograph), then you'll need to know about certain features in various graphics applications.
The best way to get your lineart ready for tracework is by adjusting the settings on your program that determine how interpolation is handled for the resizing of images. Interpolation can either be none/nearest-neigbhor, bilinear, or bicubic, assuming you have this option in your program. (May be other options I don't know about).
For best results, Bilinear is suggested for reducing grayscale work, since it leaves your art a little bit crisper and curves stay a little more true than with bicubic interpolation, which can muddle details with a slight blur.
I'll assume you have a tool with either channel or layer support.
Either set up an extra layer and put your original in a lower layer that you can pixel over without affecting the source, or use channels. With channels, you usually have Red, Geen, and Blue channels, and then you can create additional channels, which are often treated as 'alpha' when converting to png. In Photoshop, a channel with your lineart can create a 'ghost' of the original in a monochromatic color, and you can adjust the intensity of the color so it's just visible to be useful, and you can toggle the channel on and off to check how you're doing. It's always better to use an additional channel or layer when possible, so you don't have to muddy your image up with details from the original. If you don't have an alternative, then you'll want to use the Levels adjuster, brightness/contrast, and posterization to reduce your traced lines to a convenient few tones, ready to be tweaked and receive some color fills.
I hope this is helpful.