When you see a portal on your map you first need to figure out whether you can capture, attack or if it's just free already. White portals have no resonators placed, so you can just walk up and install your resonators and take control. Green or blue, though, mean a team owns it. If it's your team, you can recharge the resonators, place more, or link the portal to another. If it's not your team though, that's where the fight starts. You will be able to hack and gain items from any portal, though.