Welcome to Trainz and enjoy your ride, like a lot of us it will be a long one full of excitement.
I would start off with these following freeware/payware sites for quality content to download.
http://www.rrmods.com/
Awesome freeware locomotives and other items, large selection of payware
http://www.jointedrail.com/
Amazing payware and must needed freeware, download as much as possible.
http://www.trainzproroutes.com/downloads/index.php
Great download site, all freeware, registration required (free)
http://www.uslw.net/
High quality freeware content
http://www.guarc.us/
Freeware content, registration required (free)
http://www.jvctrees.com/index.php?pg1-cid1.html
Best trees out there for TS2009/10 - all payware, you can find other things by jankvis on DLS for free, just search JVC
For routes on DLS look up the UMR. HUGE route.
Also look up gfisher on DLS and if your computer can handle it download all of his routes - some of the best in Trainz in my opinion. Some of his other creations can be found on USLW and DLS.
That should get you started, if you need any assistance feel free to ask!
Enjoy!
Cheers,
Adam
EDIT: Check out Dermmy's routes; Clovis 1 &2 and East Kentucky found here:
http://www.checkrail.com/