You are sensing a contradiction in all this, which is itself a separate discussion deserving of its own thread. The Christian God is omniscient, omnipotent and created everything. This means that God created the entire timeline for everything "in the beginning" when everything was created, called "The Plan" so God knew everything that was going to happen, due to His omniscience, as he created the timeline of His "The Plan".
At the same time, Christians believe we all have free will. The seeming contradiction comes in the form of your personal freedom to choose to act in a manner that violates the timeline that God established. How can you have freely made a choice if God created the timeline by which that is exactly what you chose?
I am saying all of this because it is the answer to your questions. Under the free will assumption, man "sins" and leaves the grace of God, and it is from this state that man needs "saving" and thus requires a "Savior" ... and a "Saviour" if you are British. Everything traces back to the "Original Sin" which was the first sin committed by man (but blamed on women) when Adam and Eve ate of the fruit of the forbidden tree. For that, humanity was cast out from paradise to fend for themselves in a cold, hard, cruel world ... and all children born thereafter were infected with original sin. Now humanity needed "saving".
The wording varies wildly between Christian denominations. Some Christians refer to the day they were "saved" as the day they had the epiphany that Jesus is their savior. Others refer to Jesus' dying on the cross as when He saved humanity. The term "sin" means different things to different Christians which is the source of arguments, infighting, and ultimately the formation of new denominations.
So the answer to your question is that Christians believe that an all-powerful and all-knowing God intelligently created Creation per His plan and also gave us all free will to leave His "grace" and we all therefore need to be saved. Jesus is the savior, God's son, who saved humanity by dying for humanity's "sins" or alternately, so that humanity could be saved."
You'll also notice that while the above answers many questions, it leaves you with many more new ones. This is why it needs its own thread. It could even be its own board.