Drupal already includes a statistics module as one of it's core features. The problem with this module however is, that it counts each and every page access, including a substantial amount of bot traffic as well as non-unique hits generated by users going back and forth between pages. Naturally, this can skew statistics to a point where they become nearly useless.
To put things right, install Statistics Advanced and Browscap. These two modules will filter out most of the erroneous data and allow the statistics module to give more accurate readings concerning real visitor count.
Alternatively, install the Analytics module to track visitors via Google Analytics.
Tracking via Analytics has the advantage of a slightly better page loading time as no additional database queries have to be performed, but raises huge privacy concerns for readers and site owner as well.