In your POV, how will you determine if something exists or not?
If it interacts with the fabric of the universe, it exists. Thoughts exist because they correlate to synapses in the brain. Some things exist but can only be detected with the aid of technology. God does not exist, but people attribute things they don't understand to God, ie feelings of awe and inspiration (amoung other feelings), natural phenomenon like thunderstorms, floods etc., coincidences...
How will you describe "life"?
Standard scientific definition. The meaning of which is subjective
How will you describe the idea that "life" became "Us"?
Humans? Evolution. I'm trying to learn the origin of life itself right now, it's very interesting stuff. Abiogenesis
How do you describe a thought (meaning, what it is) ? and a memory?
Brain activity