Inventory limit is 2000 items once verified. So if you have 1999 items, and you do a hack where you get 5 items, it will allow you to hold the 2004 items, but until you get below the 2000 item limit you will not be able to do any additional hacks.

Deploying resonators on your faction portals does several things. It gives you 125 AP (Experience), it helps you progress your builder badge, and when a portal if fully populated (all 8 resonator slots filled) it can be linked to and from.

Firing XMP bursters at your own factions portals foes nothing. Friendly fire does nothing.

A portal is "owned" by the first person to put a resonator on it when it is white. As you are level 1 you will probably not be able to destroy enemy portals to turn them white so you can capture them - but you should be able to find portals around that are already white which you can capture. As you level up you will be able to use higher level items. Around level 5 and 6 you will start being able to take down nearly any enemy portal to capture it for yourself.

To link from one portal to another both portals must have all their resonator slots filled, you need to have a key for the destination portal (Target portal), and you need to be standing in range of the origin portal. Tap on the origin portal to open the portal view screen, tap on the link button on the lower left (It will be lit up if all resonator slots are filled) then tap the key thing on the middle right to see what possible target portals you can link to. Once you create a link - the target key will be used up and you will have to obtain a second key if you want to create another link to the same portal.