I'm cant set org-agenda to start a week on other day that isn't sunday.
As @ebpa comment set variable set custom variable via:
M-x customize-variable and then
org-agenda-start-on-weekday to set the day.
Also you have other methods.
In your emacs startup file -- maybe ~/.emacs -- put
(setq org-agenda-start-on-weekday 1)
and this will set it to Monday. It will become active the next time you start emacs. If you want to activate it immediately put the cursor behind the trailing ')' and type C-x C-e -- evaluate preceding S-expression.