What needs to be done before beginning to capture territories?

You need at least two free claim slots.
With one free slot you cant really move far on the island but you could level up your PvP level with one free slot alone.

Watch who is on the Island Chat, inspect any suspect player.

Look what territory belongs to what convergence (pull Alt-button to see it)

Now look if some player of these convergence is online and inspect him.

You should go capturing territories where none of the convergence members are online or where the other convergence member does not fit the island level.

Plan you way of capturing territories on island. Avoid capturing territories near territories of to strong players that are online and could attack you if you get near their territories.

Capturing territories with another member of the convergence on the same island is tactically better than do it alone.
It is also possible to capture it with more than two convergence member.
You need to communicate with the other convergence members, get one connive strategy of capturing the territories.
See also the numeration of the isle territories that could help here.