First let me say that it’s great that you want to come back to God. It’s great that you want to make things right, and become a true Christian.
Rest assured that God wants you back and is keen to see it happen. Being a Christian is all about having a relationship with God. Like any relationship that means listening and talking to God. So as for what you should do first - I think listening to God is a great place to start. And as for who to talk to first - I think talking to God is a great place to start.
So listen to what God says in the Bible. I’m guessing you can get your hands on one - but if not, you can read it online at www.biblegateway.com. You may know the story of the Lost Son (sometimes called the Prodigal Son) that Jesus told - it’s in Luke chapter 15 verses 11 to 32. It’s a great story - and I think one that might help you out right now.
It’s a story about a son who forgets about his father, to live for himself (verses 11-16). But eventually he comes to his senses (verse 17) and decides to go back to his father (verses 18-20). He’s expecting to be scolded, and is willing just to be a servant in his father’s house, but he’s surprised by his father’s reception (and so are we!):
‘But while he was still a long way off, his father saw him and was filled with compassion for him; he ran to his son, threw his arms around him and kissed him.
The son said to him, “Father, I have sinned against heaven and against you. I am no longer worthy to be called your son.”
But the father said to his servants, “Quick! Bring the best robe and put it on him… Let’s have a feast and celebrate. For this son of mine was dead and is a live again; he was lost and is found.” So they began to celebrate.’ (verses 20-24).
What a great picture of God’s love for us! You want to come back to God - and God is filled with compassion for you - He wants to welcome you home. And he wants to celebrate. Listen to God - and God says, ‘Welcome home.’
I think your question shows that you’ve realised - just like the lost son in this story - that you have sinned against God, by forgetting about Him. But you want to make things right. So talk to God.
Tell God what you’ve expressed in your question - that you’re sorry for forgetting about him, but you want to make things right - you want to come back to Him. You could make some of the lost son’s word’s your own: ‘Father, I have sinned against you. I am no longer worthy to be called your son.’ Ask God for forgiveness and help to live with him as your Lord and God.
Now since you’re not new to Christianity, I’m guessing you know at least a bit about Jesus. Jesus is the key to any of us coming back to God. Jesus is God’s way of making us worthy to be called God’s sons - God’s children. By trusting in Jesus, we can be God’s children again - we can have a relationship with God as our Heavenly Father.
If this is a new idea for you then perhaps you can look at http://www.christianity.net.au/god/ for a good overview. Then I would encourage you to read Mark’s gospel in the New Testament.
So, what to do first? I think listen to God by reading the Bible.
Who to talk to first? I think talk to God - thanking Him that He sent Jesus to die and rise again so that you could be called His children again. So that God could welcome us home again.
But, like all Christians, you’ll need some help to keep at it. So ask a Christian you know, or maybe go to a church or bible study group in your area. God gives us other Christians to help us live as Christians.
I hope that helps. However, if you do have more specific questions feel free to keep asking.