Ingress faction change: How much warning is given to save items

In order to level up you need to gather AP not XM (XM is energy, AP - experience), you should see an octagonal progress bar in the upper left corner.

In order to get AP you need to deploy resonators, blow up enemy resonators, build links, etc. Here are the exact numbers for each action:

Placing a resonator                       125 AP 
Destroying a resonator                    75 AP 
Placing the first resonator on a portal   500 AP 
Placing the eighth resonator on a portal  250 AP
Updating another agents resonator         65 AP
Updating your own resonator               0 AP

LINKS:
Creating a link             313 AP
Destroying a link           187 AP

FIELDS:     
Creating a control field    1250 AP
Destroying a control field  750 AP

OTHER:
Applying a portal shield    150 AP
Hacking an enemy portal     100 AP
Recharging a portal         10 AP

To look for portal in your area use http://ingress.com/intel

More here: http://ingressportal.com/research/experience-ap/

You have to go there and hack the (destination) portal to obtain the key (yes, you need to hack "friendly" portals anyway to get supplies), or trade keys with team members by dropping/acquiring them.

Also, resonators "drop" keys which are used for links when one end is blown up that you can pick up.