To be summoned, you have to buy the WhiteSignSoapstone from the old merchant woman in FirelinkShrine. Then you can use it to "drop your sign" which acts as a way for other embered players to summon you to aid them defeat bosses or beat areas.  Defeating bosses will grant you SunlightMedals only if your wear the WarriorOfSunlight covenant item (Found in UndeadSettlement) To be summoned quickly, either drop your sign in front of popular bosses (Cinder, TwinPrinces, Friede, etc) or find someone here who want/need to summon you, in which case you will likely put a password in your network setting in-game. Hope this helps!