He first created for men's sons heaven as a roof, holy Creator,
Now Caedmon goes into a little more detail with these wonders and gives us a
chronology. Also notice that humans have entered the story now, so the
wonders Caedmon lists are in relation to us—or "men's sons," as the poem
puts it, masculinely (FYI, Caedmon: women live here too).
So first God creates heaven, which Caedmon compares to a roof in an interesting architectural simile (see "The Kingly" section under "Imagery" for how this might relate to God as a king).
"First" is one of a cluster of "time" words that gives a definite sense of chronology to God's actions. (For more, see "Time" in the "Themes" section.)