I read that Fenrir killed or devoured Odin and he was dead. Is that true or false? Also, if it’s true, how does he come back to life? Is it him resurrecting himself or other gods doing the work?
The story of Odin's demise is recorded in the Völuspá. The relevant stanza is number 53. I found a translation here, which has some idiosyncrasies, but looks reliable:
Now comes to Hlin | yet another hurt,
When Othin fares | to fight with the wolf,
And Beli's fair slayer | seeks out Surt,
For there must fall | the joy of Frigg.
"Hlin" is apparantly another name for Odin's wife, Frigg. "Beli's slayer" is Frey. So, Odin is killed by Fenrir, and no more is told of him; he is certainly not among the few gods who survives Ragnarök and goes on to govern the reborn world. This is also confirmed by a later stanza, which states that
Then comes Sigfather's | mighty son,
Vithar, to fight | with the foaming wolf;
In the giant's son | does he thrust his sword
Full to the heart: | his father is avenged.
So Odin is well and truly dead, and will not be reborn (the only god to come even close to that is Baldr).