OK, so you came from symbian OS.
Every answer to every your question is in user manual of the N900.
Anyway, go to menu-app manager- tap into middle top place of the display, it will appear new menu - tab on refresh(or something like this). Then you will see many directories with programs. Chose one and search for the specific program. You can search when open the hardware keyboard and type few letters from the name of the program you are searching.
That`s it!
Hope this help, but my strong advice is - read the manual first, then use your new phone.
PS: congratulations for your new mobile-computer!!!