Spanish girl thats a great idea to learn about Pakistan and the very friendly pakistani people 
. Where you decide to stay will pretty much depend on what your plans are and the duration that you will be in pakistan.
Most foreign visitors to pakistan focus on the northern part of the country, Gilgit, Skardu, and the lower parts i.e. Kaghan, Swat etc.
A suggestion is to start your visit in Islamabad, you can visit the nearby Murree hills, Taxila and the surrounding area. If you have the time and resources you might consider spending a few days in Gilgit (you can fly from Islamabad to Gilgit).
Next stay should be in Lahore, with all its moghul architecture and the old city. Lahore is also well known for its food and specially the FOOD STREET.
After Lahore you should land in Karachi, although karachi is a relatively new city with no historical buildings as such, but you will love the overall atmosphere, the shopping malls, the beach… overall the PEOPLE ..
check www.tourism.gov.pk/ for all the basic information