How do you have a relationship with anything? You establish contact and grow the attachment bond. God has already established contact, now it is up to you to reciprocate.
How can you know for sure He exists? Talk to Him and see what happens. I can’t give you the secret incantation, nor are there spells, incense, or a special recipe to summon God; there is none.
Jesus taught us how to pray. Go into your room (anywhere to be in private and free from distraction), and talk to Him. You can say the Lord’s Prayer until you are comfortable speaking to Him fully from your heart.
Matthew 6:9-13 9 “Pray, then, in this way:
‘Our Father who is in heaven, Hallowed be Your name. 10 ‘Your kingdom come. Your will be done, On earth as it is in heaven. 11 ‘Give us this day our daily bread. 12 ‘And forgive us our debts, as we also have forgiven our debtors. 13 ‘And do not lead us into temptation, but deliver us from evil. For Yours is the kingdom and the power and the glory forever. Amen.’
Fill your heart with love, and before every thought, word, and action, ask yourself, “Is this coming from a place of love?”
Walk in love and talk to God. This is how I know. The more I do this, the more I understand life. Like I said, there is no special recipe. It takes time and effort on your part, because God is always ready to meet you where you need to be met.