People have lots of reasons for believing: Some common reasons for believing are:
1) Creation must have a Creator (the same way this computer did not just materialize before my eyes--how could the more complex living beings and systems just "appear"?)
2) Evidence found in the historical record, scientific evidence, and logic.
3) Testimony--they see radical life change in others due to God
4) Personal Encounter--people encounter God or the spirit realm in some significant way
I actually wrote a hub on it "Seven Good Reasons to Believe in God..." if you are interested in more detail :-)