How can I know if God exists?
Most people in the world think that some type of God exists. But there are still a lot of people that say "There is no God" - but how can you be certain?
Proving that something doesn't exist is impossible - all a person can truthfully say is that the thing in question probably doesn't exist based on the evidences they have seen or heard. But this does not prove that the thing doesn't exist.
An atheist is actually believing by faith that there is no God rather than believing by proof! The only other option is to agree that there is a possibility that outside of their understanding a God could exist but that they have seen no convincing arguments for God. However to say this infers that in the future there is a possibility that they could hear or see a convincing argument for the existence of God.
There are many different evidences that point to the fact that a God exists. A few of them are on this page - they don't prove 100% that God exists but they do make it very very likely.
If there is a small chance that God does exist then we should really try to find out more about Him and whether he exists as our whole eternal future will depend on what this God thinks towards us.
Absolute Morals
In order for absolute right and wrong to exist there needs to be an outside agent (God) saying what is right and wrong. Some morals are cultural and relative but there are lots that are considered as absolutes. We all live our lives as if there are absolute morals, even if we try to deny them, but the way we live shows what we really believe. Absolute right and wrong infer an outside standard or judge which would infer that God does exist.
Design
The Universe has the appearance of design and purpose therefore, if we were not looking at things from a biased point of view, we would naturally assume an intelligent designer. This is just the same as if we saw some scattered stones on a hill side spelling "I love you" - we would expect that some intelligent designer had arranged the stones in that pattern to communicate with us.