This is my speciality. I am based in Lagos.
Can you tell us where you are? There are various options depending where you are! If you are in a rural location, you may be restricted to PT (Portugal Telecom) or Vodafone (dependent on line of site to Mobile Mast - THIS IS VERY IMPORTANT). Also if you are more that 5000m from an adsl equipped exchange/sub-exchange you probably will not be able to get broadband over the phone line.
In certain urban areas you may have a broader range of options, including cable.
If you can post and idea of your location as well as what type of PC/operating System you have may be able to give more information.