Welcome, but since we don't know anything about you no one but you will know what apps will be best for you. There are plenty of threads with "Top 5" type lists in them; I suggest searching the forum for some good reading material. Enjoy the new phoneit will probably change everything!
Welcome aboard , same to me switched from bb world (9900) , there are a lot of useful app on ios , typing on the iPhone isn't that bad the keyboard is great but will take time to adapt , You can try looking in app store for essential app catagory to start with
Congrats with your new device
Congrats! I'm a defecting returner I guess... (?). I had a 3GS and then everyone around me started getting BlackBerrys, and I felt I was missing out with the whole Messenger and all, had a 9300 then 9800 then 9900, recently started missing the iPhone bad and remembering all the apps i had, so got a 4 and signed in to App Store and there all my old apps were! Dont get me wrong I loved BB (my username is from Mobile Nations CrackBerry forums), but kinda regret getting rid of the 3GS now, couldve traded it for money off the 4. Siri for me is interesting but not something I'd use so I'm happy with the 4. Anyway didnt mean to hijack thread.
+1 for WhatsApp, it's available on BB so if you have any old messenger contacts you wanna keep IMing get them to get it, I did.