What are the most sold brands and types of phones for people in their
late teens an early twenties - I want to create some games for this
demographic and I am trying to understand what phones are the most
popular and what are the best application upload sites for cell
phones?

Ok. Here's a bit of help.

Considering in the United States, Cingular Wireless is the largest
carrier with the most subscriber. I base this judgement off of their
current phone lineup, and my age (20), plus what I see most of the
teens with in todays times.

The most popular brands by far, happen to be the manufacturing
largest, Nokia and Motorola.

For the most popular Nokias among teenagers and early 20s I'd have say
these phone models:

The Nokia 6101/6102 is very popular. 
The nGages are popular, as is the nokia 6010.

For Motorola:
The most popular phone I see now, is the Motorola RAZR, v400/500/600 series.

For the other brands: Almost any of the lower end samsungs are popular. 

Hope this helps. An idea for looking @ the carriers sites, like
t-mobile, etc... look at what you'd think the majority of teenagers
could afford, even though it might not be like that, they may have a
400.00 Blackberry, but think about what they could afford, and go from
there.