"Você" always!!! a little rule to use "Senhor (a)" can be, when you talk with people much older than you, or your boss, or in a ceremony with autoritys.

"Você" is much better. Even some older people don't like to be called "senhor" or "senhora". But it's a sign of respect, no problem. Anyway, for young people "você" is definately better.

O senhor / A senhora is a formal way of address people. You use it when speaking to people you don't know or in formal situations (business, purchasing, dealing with authorities, etc...)

you should use 'você' ,is more informal and better

Oi, meu nome é Marcelo, Do you want to learn portuguese? I want learn english, so...add me on skype and let´s learn!!! my skype is marcelomirmar68 bye