There are many types of magazines in which manga is first published. If they do well and get serialized the manga can get a volume with the first chapters and sell copies on its own.
there are weekly magazines, monthly magazines, and others that come out only 3 or 4 times a year. These can also be sorted in readers age and type. it can be an action magazine like jump, or something with darker stuff, or romance and adult magazines. Manga is an artform for every age and taste.
series normally start off as a one-shot - here the author tries to convey as much of the story and characters in one volume only, then if its popular enough it can be asked to do a serialized version to submitt to de serialization meeting. this serialized version includes the first three chaptes.
stay tuned for moar.