if you hover over your neighbors island u will get a number of options , one of them is the raid option this lets you attempt to steal some of there wood/gold/rum, and also to kill some of there units if they are foolish enough to leave them out, there is also the scout option that lets you check before hand if they have any units out already or u can just go blind raiding that always ends well lol
just a personal tip not everyone does this one but try only send what your willing to loose cause it can be painfull if you loose a few thousand when your not exspecting it as a new player,
side note if you raid enough people u will get love letters people are very funny about there little havens xD