Putting (org-journal-new-entry) in the init file doesn't work, neither does setting the scratch buffer mode to journal-mode.
Kurt Hesselbart's answer works only halfway if there already is an entry for the day, instead of adding a new one it opens the last. As the function description says:
Giving the command a prefix arg will just open a today's file, without adding an entry
However it didn't work at all without a prefix arg.
Works not at all if there is no entry for the day already, starting emacs with the -debug-init option then gets the following backtrace:
Debugger entered--Lisp error: (error "before first heading") signal(error ("before first heading")) error("before first heading") outline-back-to-heading(t) show-entry() org-journal-new-entry((22041 37049 594411 222000)) eval-buffer(#<buffer *load*> nil "/home/anon/.emacs" nil t) ; Reading at buffer position 7751 load-with-code-conversion("/home/anon/.emacs" "/home/anon/.emacs" t t) load("~/.emacs" t t) #[0 "\205\262