What were the first creatures created by God?

Birds and fish (Genesis ch.1).

Note:

The creation of birds is mentioned both in Genesis ch.1 and ch.2.

Rashi commentary (2:8) explains that Genesis ch.1 and 2 form a single set of events, as chapter 2 expands the narrative of ch.1 with further detail.

An attempt to split the text artificially has been perpetrated by people such as Wellhausen (the father of modern Biblical-criticism, 1844-1918) who suggested attributing the narrative to various authors, despite the Torah's explicit statement as to its provenance (Exodus 24:12, Deuteronomy 31:24). Wellhausen's claims have been debunked one by one, as Archaeology and other disciplines have demonstrated the integrity of the Torah. No fragments have ever been found that would support his Documentary Hypothesis, which remains nothing more than an arbitrary claim, whose falsehood has been pointed out:

http://www.apologeticspress.org/apcontent.aspx?category=6&article=1131(a Christian author)

http://www.whoreallywrotethebible.com/excerpts/chapter4-1.php

http://www.pearlmancta.com/BiblicalcriticswrongRShlomoCohen.htm