You would need to get the metro from Porto Airport to Porto Campanha Railway Station and then a train to Braga.
Extract from Timetable here:
http://j.mp/bWQd0g
The Metro Line E trip would take about 35 minutes.
The 2010/2011 Porto Metro timetable can be found here:
http://j.mp/aceNyh
An Airport Transfer might also be another option.
These guys are quoting € 114.00 for Porto to Braga for 4 people.
www.low-cost-transfers.com/porto-airport/
