How To Customize Code Files

Hi!
I want to customize my classified child theme to certain level where I’ve the following changes:
1) I want to display my category and subcategory just like flipkart from the top. Whenver user clicks on a specific subcategory
I would like to show him the adds for that particular subcategory.

2) in the search panel distance is coming as a search criteria. I want to remove the same and add places instead of distance.
Like pakistan, kolkata etc. It should be a textbox with auto complete feature.

3) Whenever I enter any category from home page, a widget comes up which shows all the categories on the left hand side.
Instead I want to show all the subcategories for the selected category in that widget.

Can you guide me through the files please?