The best way to show your friends that God exists is through your changed life, and through your unconditional love for them that God will give you the power to give to them. We can argue and argue about what proof there is for or against God's existence, but no one can argue against the evidence of a changed life, a changed heart and mind. Skeptics will rationalize answered prayers as a coincidence, and will argue the design of creation as a by product of billions of years of evolution.
But one thing you may ask them is, was any person there when the universe began, whether it was created by God or by the big bang? Both beliefs take faith to believe, because no one was there to witness either possibility.
But still again, the best possibility of shoeing your friends that God exists is to let His light shine through you. If God were to let His light shine on us directly from Himself, with us seeing Him, we would die. Therefore, in order for the unsaved to behold the evidence of His existence, He must use us, and He will only use us to His glory if we are willing to let Him do so. Will you let Him do so? Then trust that He will prove His existence through the light of His love and holy character that will shine through you :)
God bless and keep you,