Hi and welcome to the maemo community.

I could write pages and pages of stuff to do for a new N900 user, especially a linux user. For most of the things you'll need to search. I can however braindump some keywords for you to get started, in order of how adventurous you feel

1. Community Seamless Software Update for the latest bugfixes and features
2. Easy Debian for full desktop apps on your N900
3. Preenv for hours of gaming
4. Powerkernel for mobile hotspot, bleeding edge wlan driver, hostmode etc
5. busybox power for less terminal frustration
6. FCamera for unseen power over your photos (install version from testing)
7. CSSU Thumb for unparalleled speed
8. Nemo mobile for a peek into the future
9. Ubuntu 12.10 for a real desktop in your pocket

Before doing any of this, however, learn how to reflash so you can bail out of any bad situation, and try backupmenu so that you can play around without fearing of losing data.

Lastly take good care of your USB port or as senior members advise Fix it before it's too late. A fine way to take care of it is to buy an external charger and some spare batteries

Have fun!

PS. I forgot to mention, ssh is the best way to administer a remote linux box be it the N900 or your desktop. Even X Forwarding works both ways (not for all applications)

see this too:http://talk.maemo.org/showthread.php?t=87519