I can press o but it creates a new window and opens the file there. I want it to open in the same window. At least it should display on the same window each time open some file but it creates new windows for each file I open. Also is there a way to make it open as a vertical split and not horizontal split? Where can I find the complete documentation/cheatsheet of all possible commands in 'Netrw Directory Listing' mode?
For the answers to your other questions, see
for all the documentation,
for using a vertically-split window, and
for a listing of the normal-mode commands (maps).
Here is quick reference from Help