Deities of Faerûn
It is well known in Faerûn that those who die without a patron deity to send a servant to collect their souls from the Fugue Plane at their death will spend eternity writhing in the Wall of the Faithless or disappear into the hells and infernos of devils and demons.
Having a patron deity does not preclude a mortal from worshiping other deities as need arises. A wizard heading into battle is well advised to give obeisance to both Tempus and Mystra.
The first step is choosing your patron by Pantheon:
Sins and Penance
Some members of the clergy believe their deities watch over every act, thought, and consequence of the deeds of every mortal worshiper. Most priests, however, see their deities as judging mortals only on deeds or on acts plus obvious intent rather than ultimate consequences.
A cleric or druid who commits a minor offense against her deity or ignores portions of the deity's dogma is guilty of a sin. He has to do some penance appropriate to the seriousness of the sin in order to remain in good standing with the church, other clerics or druids, and the deity. Paladins, rangers, and other divine spellcasters are held to this standard (to a less exacting degree) also.
Typical penance for lesser infractions includes spending an hour in prayer, making a small monetary donation to the temple (1 to 10 gp), performing minor duties in the temple (which vary by religion), and so on.
Penance for moderate infractions includes spending anywhere from a day to a tenday in prayer, making a moderate monetary donation to the temple (100 to 500 gp), or going on a small quest for the church (a short adventure).
Penance for major infractions includes a month or more of prayer, a large donation (1,000 gp or more), a quest, and possibly an atonement spell (which might require its own quest).
Continued abuses of the church's dogma may result in a divine spellcaster losing his class features (but not any class-related weapon and armor proficiencies) until he atones for his sins.
It is possible for a cleric, druid, paladin, or spellcasting ranger (or any other divine spellcaster) to abandon his chosen deity and take up the faith of another deity. In doing so, the divine spellcaster loses all class features of the abandoned deity. To progress as a divine spellcaster of another faith, the character must go on a quest for his new church (often the recovery of a lost item of some importance to the deity), then receive an atonement spell from a representative of his new faith. Once these two conditions are met, the character becomes a divine spellcaster of the new deity, and if a cleric, he chooses two domains from the new deity's repertoire. The character then resumes the class features lost from leaving the old faith (so long as they are still applicable - turning or rebuking undead ability might change, for instance).
About Planar Domains
A planar domain counts as both of a cleric's domain choices. The granted powers of a planar domain are more potent than those of other domains, and each level offers two spells from which a cleric may choose when preparing spells. Each day, a cleric with access to a planar domain chooses one of the two spells available to prepare in his domain spell slot for each spell level. Unlike other domains, planar domains each have an alignment requirement that must be met by a cleric who wants to access the domain.
Though some of the domains below mention deities that can grant access to their followers a cleric need not select a specific deity to have access to a planar domain. A cleric who devotes himself to a specific alignment (LG, NG, CG, LN, CN, LE, NE, or CE) rather than a deity can select a planar domain in place of his two normal domain choices. If the DM wishes, she can make domains specific to other planes, using these as representative guides.