This is quite simple.
You need to get the Light Rail Metro from Porto Airport to Campanha Station via Line E.
See:
http://www.metrodoporto.pt/pagegen.asp?SYS_PAGE_ID=871913&id=154
and
http://www.metrodoporto.pt/document/827457/874411.pdf
From Porto Campanha there are trains between three and five times an hour to Espinho from 05:09 until 01:00, Takes up to 25 minutes.
For timetable in pdf format See:
http://www.cp.pt/StaticFiles/Imagens/PDF/Passageiros/horarios/urbanos_porto_2007/aveiro_linha.pdf
