Just look up the wiki entries on adding extras, extras-testing & extras-devel. Exercise caution when Installing from these repositories, loads of stuff in devel is broken/half assed/half finished. (but it is also where a lot of the cool stuff is).

The BEST advice I can give you is learn to reflash NOW! It is the first thing you should figure out. It will let you fix the N900 when you break it (which you will).

In addition to that figure out how to use the program 'backup menu'.

Backup menu + reflash will save your *** literally 100s of times in the next couple of days as you figure out what how to use your N900.


...

Also, do not bother with NITdroid or multiboot. It is a world of breaked n900's.