You have to leech all of the available files of a torrent until you have all of it, for a complete snatch. You can choose to only grab (snatch) one file of a collection, but then you will still be in the "leeching" category since you do not have it all (doing so keeps open one of your download slots). In the meantime, you will be seeding the file or portions you already have, if someone else is leeching from you.

The uploader gets ratio credit for everything they actually seed (share or upload); often it will be more than one complete copy as not all leechers can connect to each other. It does not matter who is the first leecher, other than being the first can give you more seeding opportunity for a better ratio. If the uploader put three files in a torrent, but only one leecher showed up and took only one file, the uploader will get credit for only that one file, because that is all they seeded or shared.