i have a blackberry curve 8520 and i tried setting up my wifi conection and it siad it was succesfullly conected but when i set up my connections and i connect to my wifi, it says its on but it have a white minus symbol next to it. how can i fix this? and how do i know if my BB is using my wifi connection or using the internet from my phone provider? because i have no data plan and if i use there internet they will charge me. i just want to use my internet from home.

Likely it is the fact that you do not have the BlackBerry data plan that is your problem. Some (maybe most) carriers require the BlackBerry data plan even for wifi. BlackBerrys are different.
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do i have to go to at&t to get a blackberry data plan?

What carrier do you use today?
If you use AT&T, and it is an AT&T branded BlackBerry then yes.