or little bit informal and simple: Nie mam tu netu.
Don't be so serious. I am graduated in polish philology and I use this experssion - like: Mam net / Nie mam neta(u) / Net mi działa / Net mi nie działa etc. - very often. Of course it is colloquial, but why should we always speak the language of Mickiewicz only.Don't use this second expression presented by Jordi. It is very colloquial and I have a lot of doubts if it is even consistent with Polish grammar. First of all, if people use it they tend to say "Nie mam tutaj neta which is not correct . Anyway, just refrain from using it and it'll be ok. It's much better to say something in a normal and a correct way instead of trying to be informal or colloquial by force.
It depends. While studying the other languages, I am very interested in the informal expressions. Becouse in fact, if I want to understand the everyday language, I need them more then the formal frases. Moreover, if I want to contact with the my friends from other countries, I prefer using - f.e. in email - more colloquial words.meta_fora. It doesn't really matter what kind of major we study and how we do speak in everyday life. It is the matter of the way of learning others Polish (or just translate simple sentences). First of all, a person who learn any foreign language should get to know formal expressions and words. The colloquial way of speaking, in my opinion, should be taught when a person has bases in a foreign language. I thing that in fact it comes spontaneously while going deeper and deeper in a language. Then he or she would be able to feel what kind of expression should be used in which situation.