To be summoned simply use this object to put a sign on the ground, and then stay within the area (usually between the bonfire and the boss) until someone summons you, you can take the chance to gain a few souls in the meantime (this might take awhile since it depends on the level of players within your area and level range that are in human form).
To summon (either other players or NPC) you need to be in human form.
If you have one of this items you can use it to gain humanity and you can return yourself to human by using a bonfire ("Reverse hollowing"), after that simply look for a summon sign on the ground to summon someone.
Alternatives ways to gain humanity are to kill bosses,helping kill a boss while summoned, or killing a certain amount of enemies (that used to be human) in an area (before you defeat the boss).
Keep in mind that being in human form leaves you open to be invaded by other players.
The game doesn't has a tutorial about this things so it's not that odd that you wouldn't know about them, I only knew about it myself because someone explained me the Demon's Souls mechanics after my first attempt at that game.
Here's a page with more details on the multiplayer aspect of the game.
With that said it should be noted that some people have issues summoning/being summoned on the PC version for some reason, so don't be too shocked if you get "Summoning failed" when trying to summon another player (this won't be an issue with summoning NPC's).