Look for genres you enjoy, read reviews, and start with popular series. That's a good way to pick.
First, think about what themes or types of stories you like. Then, check out recommendations on manga forums or from friends. Also, look at the art style - if it catches your eye, it might be a good one to try.
You could start by looking at manga rankings and awards. Also, see if there are any manga that are based on franchises or properties you already like. Another tip is to browse through samples or previews to get a feel for the story and art before committing to a full read.