Odinne, one of the benefits of finding a bike at a local bike shop is that you can actually "shop". You can try out different bikes and see how it feels getting on/off, maybe even test ride it a little in the parking lot.
You can get an idea of bike size, wheel size, brands, etc. Then you can search Craig's list and other sources of used bikes and you will know what you are looking for --and you can price shop that way.
I would recommend against buying a bike that you haven't tried first... so try them out at the bike shop and if you can't afford their prices, go price shopping elsewhere.
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"I never made "Who's Who"- but sure as hell I made "What's That??..."