Step 1: find your scan!
Self-explanatory; however, there are several things to consider when picking which scan to color, such as:
The image's attributes: if you're new to working with b&w images, I highly recommend using big scans, as it will be easier for you to see what you're doing. Try to pick clean images which have no artifacts (graininess from screentones, dust/folds from improper scanning etc.) for starters, until you get the hang of cleaning.
How ready you are: an important process of coloring scans is the cleaning, which can prove difficult for those new to it. For starters, pick a decently-sized colored image and try your hand at cleaning that before moving on to b&w images; once you've gotten the hang of it, pick a decently-sized b&w image and clean that. Eventually, you'll be able to tackle more complex images, but you must be honest to yourself about your level in order to progress.
How much time you're willing to devote to it: depending on the two factors above, coloring a scan can take 10 minutes or 5 hours. You shouldn't rush yourself, as this can easily lead to mistakes you may not be able to spot at a quick glance-over. Make sure you put aside at least an hour if the scan is big, so you can have enough time to color it and look for errors.
Ideally, the scan should be of something you enjoy looking at and are familiar with. Furthermore, try to find a few reference images for the color palette before you get to work. Why a few? Because colors usually vary in colored manga art - for example, a character may appear blonde in an illustration but actually have brown hair in the story.
Optional step: positioning the scan
If the image is tilted, you can position it according to your wants. Same goes if, say, you only wish to color/focus on a single character in the image of a group and crop out the rest. Be careful not to cut out any important parts when cropping! Think about what you can delete later and what you can reconstruct: maybe it's easier to delete a character's sleeve than to reconstruct another's arm.
On towards step 2, where we'll see how to clean the scan before coloring it!
