How can you be sure that you believe in the right God? There are many different religions and philosophies all over the world that believe entirely different things about who/what God is and what He expects of us. There is Christianity, Islam, Judiaism, Hinduism, Budhism, just to name a few.
I believe that you can find the truth by simply asking God to reveal it to you. I believe that the God who created the universe loves the whole world and wants everyone to learn the truth about Him (See II Peter 3:9 in the Bible). You can simply ask Him through prayer to give you what you need. HOWEVER, MAKE SURE THAT YOU ASK HIM IN COMPLETE HUMILITY. Do not demand anything of God. Do not test Him. Simply beg Him to help you see the truth through His mercy, because you need Him, not because you deserve Him. Do not ask for a sign. Ask Him to give you whatever you need. He knows what you need better than you do.
24 “The God who made the world and everything in it is the Lord of heaven and earth and does not live in temples built by human hands. 25 And he is not served by human hands, as if he needed anything. Rather, he himself gives everyone life and breath and everything else. 26 From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands. 27 God did this so that they would seek him and perhaps reach out for him and find him, though he is not far from any one of us. 28 ‘For in him we live and move and have our being.’[b] As some of your own poets have said, ‘We are his offspring.’ (Acts 17:24-28)