Hi GreatNews. Nice username, what is the story behind that?
In answer to your very good questions:
Why are we (humans) on this planet earth..? Where have we come from? Did someone create us and put us on here? And for what purpose?
We are here for one purpose only, and that is for God's pleasure. Yes, you read that correctly, He gets actual real live pleasure from us humans. It's sometimes hard to see why, when we ruin our planet and often each other, but I would think the best place you could view it from is from that of a parent. Are you a parent? If so, you may get a small glimpse of the pleasure parents get from their children and extrapolate that to the pleasure God gets from us (his children). This isn't a case of us being some form of amusement or pantomime entertainment, but the sort of pleasure a person gets from a meaningful encounter and a deep, personal relationship; a lasting love. Genesis 1 says we were made in "God's image", though we are not equal to him, obviously. He didn't need us for friendship, but he chose to create us despite being self-sufficient already. Colossians 1 says all things were created by him, and for him. Revelations 4 says God is worthy to receive glory, honour and power, and that for him all things were created.
Does anyone has any plausible answer? With some degree of certainty, and not just some blind faith or nothingness.
It is not socially acceptable to say you have certain faith, which is quite ironic because the people insisting you cannot speak of such certain, do so with certainty that their way is right (see the irony there?). I am certain, not with some blind wishful thinking, but because I have diligently searched and questioned, and because God is true to him word when he says "if you seek me you shall find me".
My search has involved looking into the credibility of the bible (yes, it really is very credible, historically, archaeologically, prophetically, to name but a few - though again, you will always find people insisting that can't possibly be the case). No one has ever been able to evidentially provide a serious case against the bible which stands up to critical scrutiny.
Then there's my personal experience, which nobody can take from me. It is the hope and the assurance that; no matter what I experience in this life, God has a hand in it all and has a purpose. Just because I don't always know what that is, doesn't mean it's not there. And there is the promise of not just a perfect eternal life, but also wonderful real help in this life too. It is about a relationship with the one who created me, and there is nothing on earth like it.
Atheists/Humanist do not believe in any sort of deity, religious people believe in a deity but differ amongst themselves what sort of dirty it is.
That's exactly right, and they can't all be right, can they? One god says you have to work harder and give more, another god says you can just do your best and wish for a good end, atheists say we all live and die then nothing. Some religions say you are born repeatedly (reincarnation) and the bible says it is appointed unto man to live once and then die. You can sincerely believe that you are able to fly, but if you jump off a bridge you'll find that you can't. No matter how much you believed you can. So it's important that you trust in something (someone) utterly trustworthy, and not of yourself when it comes to your eternal life. The bible is so clear, we can't buy our way to heaven, we can't earn it by our good works, the only way to the father is through Jesus, the son. He took our penalty and clothes us in his righteousness. It is a free gift with no strings attached, but like any free gift you have to choose whether you will accept it or not.
Surely, we didn't just appear on this planet for no reason or a purpose.
You're exactly right, we didn't. God created every living thing, and Psalm 139 speaks of how he lovingly watches us grow, and how we can't escape him anywhere in the universe, he is there.
What happens to us when we die?
We will be present with God, or we will not be. This is not a changeable status once you're dead. Sometimes people think that a loving God wouldn't turn people away once they regret their choice after death, but sadly God makes it clear there are consequences for sin, and if you don't accept Christ's gift of Jesus' righteousness, they you will be trying to be in God's presence under your own steam, which does isn't possible. The consequence of sin is death (because God is completely holy and we are not) but, as Romans 8 tells us, the free gift of God is eternal life (HIS holiness given to us).
Our bodies decay but what happens to the soul or consciousness?
The soul lives on eternally. We were created to be eternal beings.
Yet to find an convincing answer that makes sense other than there is a life after death.
Romans 1 says that the evidence of God is all around us. It is in the kindness of a stranger, it is in the beauty of nature. It is in the miracle of a newborn baby, it is in the still during a storm. As a result of this, the bible says man is without excuse. There is an inbuilt knowledge of God that is making you ask all of these questions!
Learning about various religions and non to have better understanding for the peace of mind.
Well its good to ask your questions but remember they all contradict. The main difference is that they all expect a level of hard work from you, without really ever knowing if you've done quite enough. Other gods are aloof and distant. Whereas with God, we have the way paved out for us by way of a free gift offered, a life guided by the hand of a God who IS love, and who loves you more than you could ever imagine. A Creator who sees everything about you that you don't like and loves you in spite of it.