I know this is a question I hear from my kids all the time. They want to think heaven is that beautiful spot in the clouds where the streams of sunlight originate as they shine through the clouds :). Here’s a great explanation from the YouCat (Youth Catechism):
If the Father is “in heaven”, where is that heaven?
Heaven is wherever God is. The word “heaven” does not designate a place but, rather, indicates God’s presence, which is not bound by space and time.
We should not look for heaven above the clouds. Wherever we turn to God in his glory and to our neighbor in his need; wherever we experience the joys of love; whenever we convert and allow ourselves to be reconciled with God, heaven opens there. “Not that God is where heaven is, but rather heaven is where God is” (Gerhard Ebeling). (YOUCAT question 518)