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:
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 deitiy to have access to a planar domain. A cleric who devote 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 hid two normal dormain choices. If the DM wishes, she can make domains specific to other planes, using these as representative guides.