How to customize my child theme?

I was hoping someone might help me.

I used to have a theme all set up but it got old and i had to pick a new child theme. It is similar but it has a few different things that i would like to change, but i don’t know how nor where to do it. Maybe someone here might give me a hand?

The Changes i need are:

1- I need back ground to be all white. See image attached 001
2- I need to remove the blue square of the object box bar. See image 002
3- I need to make the footer look like the one of the image. See image 003

If anyone could help me i would really appreciated!

P.S.: Please excuse my English but I speak Spanish