Pakistan's official language is English and many people in big cities can speak it. Urdu is the national language and is spoken throughout Pakistan as lingua franca. In addition to Urdu most Pakistanis speak their regional languages or dialects such as Punjabi, Sindhi, Pashto, Balochi, Saraiki, Shina, Burushaski, Khowar, Wakhi, Hindko etc.
English is the official language and is used in all government, educational and business entities, and is also understood and spoken at varying levels of competence by many people around Pakistan, especially by the upper classes and people who have gone through higher levels of education.

Here are some basic Urdu phrases. If you can speak those, it will delight everyone you meet:

Respect

Pakistanis pride themselves on their tradition of hospitality to guests (mehmanawazi in Urdu, milmastia in Pashtu). Just a greeting of Salam Alaykum will get you far in endearing yourself to people. If you are travelling outside the big cities like Karachi, Lahore and Islamabad it is advisable to learn some basic Urdu or a regional language.
Do not make silly jokes about religion, women, sex, or alcohol. You will cause offence and it is something you're really ill advised to do.
Just respect and observe. Pakistan is a conservative country and it is advisable for women to wear long skirts or pants in public (Pakistani women wear the traditional shalwar kameez). Dress codes for men are more lax although they should refrain from wearing shorts in public. As well, showing someone the sole of your feet or shoe is considered an insult, and can lead to you being considered disrespectful.