To be included in WoS: a journal has to have been published for at least three years. Beyond that, the main criteria are: timeliness (must deliver issues on schedule), ethical publishing practices, full text in English (or at least have bibliographic information in English), peer review, and international diversity (are editors and authors from around the world). They also look at the citation history of articles published in the journal.
The full selection criteria are here: http://wokinfo.com/essays/journal-selection-process/. [Note that the criteria are discussed in detail starting with the "Overview of the Process" section.]
If you wish to have a journal added: If you are on a journal editorial board and wish to have your journal considered for inclusion in Web of Science Core Collection - you can submit a request at: http://wokinfo.com/publisher_relations/journals/