According to Islam, God is formless, beginning-less, end-less, creator of ‘things’.
So the question who created God is invalid, since the things of cosmos are created by him and he doesn’t qualify as ‘things’ in the sense that we comprehend ‘things’ of this physical realm.
The creation itself is commenced by Allah, and created-ness is the attribute of creation thus Allah can’t be created and it is illogical to ask ‘who created God’
With regards to God existence, everything around you is a proof of God existence. Your eyes,your mouth, your brain, your DNA, the fingerprint.
A clever person once posed a question to an illiterate Bedouin “How do you know that Allah exists?” The Bedouin pointed his finger towards a camels dung on the sand and asked that man “What is that?” The man replied “a camels dung” The Bedouin then replied “if that dung is a proof of camels existence then this entire universe is a proof that a Allah (swt) exists.”