Television newsreaders are normally very experienced journalists. They're senior to many of their colleagues because they normally have editorial input. (i.e. they help to determine the way that each item should be presented). So, you're chances of becoming a TV newsreader without extensive training (usually at degree level) and experience as a journalist are almost nil.
Newsreaders on radio, however, are often part of the continuity team. Although many are also journalists, this is not always the case. See here:
http://www.learndirect-advice.co.uk/helpwithyourcareer/j obprofiles/profiles/profile829/
What you really need to think about is what newsreaders do when they're not reading the news. They certainly don't get a full day's pay for reading a few short bulletins! Most newsreaders are working throughout the day to help compile the news broadcasts (which is why experience in journalism is essential). Others (particularly in radio) will be working as production assistants or doing jobs like taking phone calls to compile the traffic reports.
Chris