This depends on the style of manga and your own goals. There are a lot of hubs and websites that can give you styles and basics to work off of. Sometimes people find looking at a manga character and drawing them to be good practice.
The best way to start is to draw a simple form, a cross. Two lines, one longer than the other, and they must cross. Then, using the upper portion of the cross, draw a "head" shape, whatever suits your character. Then go to the smaller line, and add general details (arms, shoulders), then draw an outline of the body. After that, the details.