If I wait - Or more accurately, “truly I expect that the grave will be my home.” The word rendered “if” (אם 'ı̂m) is often used in such a sense. The meaning is, “I look certainly to the grave as my home. I have made up my mind to it, and have no other expectation.”
The grave - Hebrew שׁאול she'ôl. It may mean here either the grave, or the region of departed spirits, to which he expected soon to descend.
Mine house - My home; my permanent abode.
I have made my bed - I am certain of making my bed there. I shall soon lie down there.
In the darkness - In the grave, or in the dark world to which it leads; see the notes at Job 10:21-22.