I wasn’t sure if you meant generally, or in terms of evolution.
Generally, something that is alive respires- we all take in oxygen- plants do it, we do it, and bacteria do it- everything does. Viruses don’t and that’s why they’re not said to be a ‘living thing’.
In terms of what ‘made’ us alive, it’s very difficult to establish how the first life on earth came about and there is a lot of debate about it because it was so long ago that there isn’t much evidence left. It’s thought that it started by the ingredients coming together to make single-celled organisms (like an amoeba and bacteria) and, slowly, these evolved into more complex organisms with multiple cells. The original life would have been very simple and wouldn’t have been very recognisable as something we know today as we are much more complex. We, and even simple organisms now, are very well adapted to our environment- but this has taken a long time of evolution. It would be very interesting to be able to look back and see what made the first life, but unfortunately we don’t have time travel! (Personally, I’d like to try out living in the 1950s, it looks fun!)
I hope this has answered your question? It’s very deep and everyone has their own personal ideas about it. If you want to know any more, let me know!