welcome, do you know bicycle mechanics and would you be working on eveything yourself? If so I would highly recommend buying used from craigs list or ebay. if your not so mechanically gifted then definately walk into a bike shop and purchase new. the shop will probably offer some "follow up" maintenace incentive and help you select the proper size frame.
what type of bike are you interested in? fat tire (mountian), skinny pizza cutter wheels (road bike), or something in between (hybrid). Do you want gears or not?
2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
1997 Trek ZX6000, 6061w/manitou spyder, xt/xtr, time atac