Hi Pianoman81 -
Well I believe in God because I believe the Bible, and what the Bible says about God. I believe I met Him in my heart when I asked the Lord Jesus Christ to be my Saviour.
So one of my 'proofs' would be that I have an experience of knowing God, of having daily fellowship with Him and of seeing prayers answered.
Other proofs of God's existence that people mention are:
The religions seen in all the various cultures, historic and current, of the world. Men seem to have a need to worship someone or something. Why do all civilisations have this spiritual need if it wasn't put in them by a Creator Who wished to be worshipped?
(Also our consciences, right and wrong, morality etc. How could evolution have developed that, which is actually a handicap if you're just an animal that can do what it wants.)
The fact of the universe's existence. As I think Theland said, 'cause and effect'. Something had to 'cause' the universe. So either the cause was a 'big bang' or God.
(Cause and effect only applies to something that has a begining, as God always IS He doesn't have a begining, nothing 'caused' Him, He always IS, self existing.)
The order, purpose and harmony seen in nature. The extreme depth of design in everything suggests that there must have a been an intelligent Designer to have made it all.
Even looking at the incredible enginerring in something as simple as a single cell could surely not be explained by something as random as evolution.
Hope that is at least some help to you.