Many Christian hymns talk about god as being "eternal", meaning he has always existed.. However the bible does not actually make that claim.
I had a look through the Bible and could find these words relating to God being eternal:
Hebrew "kedem" meaning ancient, translated in most bibles as "eternal". Deut 33:27
Hebrew "olam" meaning literally concealed, closest to eternal I could find Isaiah 60:15 but does that relate to the messiah rather than Yahweh?
Greek "aionios" as in "eternal life" Matt 19:16 when clearly eternal life here means something that has a start rather than being preexisting from eternity. Also used in "eternal judgement" Heb 6:2 which must mean everlasting from the point the judgement is made.It seems that at the time of Jesus this word was used to denote a fixed period of time Matt 25:46 everlasting punishment clearly can not start from eternity it has to be endless from a fixed starting point.
Greek "aidios" closest to the idea of everlasting as from all before times Rom 1:20
Philo used it to mean "endless"
The septuagint translates olam as aidios whih is interesting but not conclusive as to meaning when the OT context rules out pre existence. The only usage of aidios meaning eternal in the sense we are talking about is in Romans 1:20 which I am prepared to accept talks about God's power being eternal, but its only one verse on which to hang a rather large idea and its a passage about fidelity to God rather than idol worship, its not addressing the nature of God except as a byline.