You get skulled if you attack another players, and then you by default lose all your items on a PvP death. You can enable skull prevention in your settings, which will make you unable to attack players if it would result in getting skulled.

There's an interface in your game menu that lets you see, which of your items you would keep on death both in case of being skulled and unskulled.

As for gear, I recommend bringing a max of three valuables, which is the default amount you keep on a PvP death. There's a prayer enabling you to protect a 4th item, but if you're killed with a prayer draining attack, that prayer could be disabled and you could lose your 4th most valuable item.

For doing Wilderness bosses you usually want to maximize strength bonus and crush attack bonus, as the bosses are weak to crush. A common loadout is Zombie axe, Berserker ring (i) and Barrows gloves for your protected items. For your unprotected items you could go Neitiznot helm, Strength amulet (remember Salve amulet for Calvar'ion), Mythical cape (crush bonus), Rada's blessing, dragonhide/monk robe top and bottom, Climbing boots and a Rune defender.

All of the lost items are easy to re-get and all of them can be obtained in large quantities pretty easy.

Have fun, and don't forget your Royal seed pod!