It's both amazingly simple but incredibly hard to have. God says, and you believe His words. There's nothing else.
It can be as simple and as automatic as reading and believing His promise not to break a bruised reed (a tender conscience pricked and convicted He will not cast down). Or His promise not to cast out any who come to Him.
But it can be as hard as believing that you are really in His grace, that you really do want Him, that you really are one who is called.
If it is hard for you, try to keep it simple. Do you want to be saved? Do you see your hopeless state of sinfulness? Do you see Christ as the only way? What else is there? Desire to come, and trust God to enable you to come. Then come and don't look back!