Why is my sony ecrisson xperia arc S mobile data not working?

[AC Question]

I bought a sony ericsson xperia arc S about 4 days now. I couldn't use the date. I use to on it alright but no signal, I use wi-fi to set up all i need but when i want to download any apps from google, they keep telling me Data Connectivity Problem. Sometimes, they give me a sign that security warning. There is a problem with the security certificate for this site. this certificate is not from a trusted authority.
I need a help to operate my phone Please.

 

[B. Diddy]

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