Check out Romans 10:10 - it tells us that it is with our hearts that we believe.
We're called to love God with all of our hearts, to believe in him wholeheartedly, to trust and have faith. Perhaps, you're thinking that it is easier said than done (and there is an element of truth in that) but you can make a start.
Think about what could be an indication of whether someone really believes. I think my prayer life and yours shows how much we believe. Someone who prays a lot and checks out all the important stuff with God believes a lot - at least, that's what I would argue! Someone who prays less probably believes less. If you don't pray much or, for example, you only pray when you want something (we've all been there!), try praying more and try praying for all areas in your life. - the small stuff as well as the big. Talk to your God about anything and everything - he loves to hear from you.
Believe in him with all your heart.