One of the great concepts of Hebrew and Christian scripture is the free gift of God to humanity. What God offers is free.
The metaphor is food but Yahweh is more deeply speaking of paying attention to Him as creator, what gives us life and what takes away life.
If we think we are evil, there is a fairly straight-forward response here in Isaiah 55. Stop being evil. Come to Yahweh so He can have mercy on you.
Read Isaiah 55:1-7 as a prayer, personalizing it as you go along. Read it something like this: Yahweh, you said all who are thirsty can come to you and drink freely. I am thirsty for truth, for the freely given mercy you give. I am hungry for you. I have gone my own way and ended up empty. Fill me today. I have done evil and not obeyed your ways. Thank you for your mercy upon me and allowing me to come to you and to stop doing evil.
Remember that true fasting is confession. Read Isaiah 55:6-7.
As you consider giving a free meal to someone, remember that God freely gives of all He has to you.