Faith is believing God. When you call on Him and ask Him to save you, and deep in your heart you sincerely tell Him you will receive Jesus as your Savior. God wants you to KNOW you are saved because He said He would save you. That is believing God. He wants you to trust Him to do exactly what he said He would do - and He said He would save you if you called on Him.
Some people have faith in a dream, some in a vision, some in feelings, and others in emotions.
We are born again by the Word of God. If faith is believing - then we must believe the Word. I am just as sure I am saved, as I am sure the Bible is the Word of God. When you have heard God's plan for your redemption, you call on God and ask Him to save you. You believe God and trust Him to save you because He said He would.