why buy the cheapest main board/processor possible?
is this a one time complete purchase or are you planning to upgrade later,
like you may use onboard graphics now but want to upgrade later.
a regular DVD drive might be nice but are you going to want to watch blueray movies on the computer? are you allowing case space and power requirements to add new drives later?
what about sound? you using onboard sound or getting a decent sound card, you going surround sound or just stereo...
As berry has been trying to tell you, there is no generic guide for teens...
as an example, when I was a teen I pretty much used my PC for creating small programs in Qbasic, and then moved on the visual basic, (version 3). I didn't really need any power at all so was happy with a really cheap machine.
I played in a band with a friend who was really into music, so his rig was a bit more powerful, he ran virtual studio software on it, had surround sound set up so that he could get the most of out his music, had dedicated midi controller cards so that he could plug his keyboards into it. a nice big monitor so he could see what was going on.
had another couple of friends really into gaming, so they had computers with decent graphics cards, and had bought network cards so that they could take their PCs to each others houses and connect them together, (this was in the days of BNC coax cables and terminators...)
there is no BUILD GUIDE FOR TEENS as if you talk to ten different people they are likely going to want to do ten different things on it.
ranging from checking face book, to creating the next face book.
listening to music through to making music.
watching videos, editing videos. playing old games, playing the latest games.
your $600 machine you've spec'd up is wasted on the teen who literally just checks facebook a few times a day.
but it won't be nearly enough for the teen who must be playing the latest releases.