I am researching and have been researching ball pythons for the last few weeks

I am interested in getting a baby, but what is a baby i mean like how old from 0 days to what?

And i have a 20 gallon standard fish tank and have read the glass setup part. I do have an extra uth for smaller tanks 5gallon for plastic but don't want to buy a 10 gallon and lid and all that so how should i set up a one shelf tank.
How big should the sweater box be?
How long should I keep him/her in there?
Is a uth ok, and also what are the other requierements?
Last q, how can i keep the snake from escaping?

THanks All comments appreciated,
Marcus