To actually type the characters, you can use C-x 8 with the appropriate prefix, e.g. C-x 8 ' e inserts é. Get help for C-x 8 with C-x 8 C-h. You can also use an "input method", which makes it easier to type certain characters. The only one I've ever used is TeX, because I know LaTeX and I generally don't need non-ascii characters.
Emacs should automatically choose an encoding that can encode all of the characters in your file. If it doesn't, you can change the encoding with M-x set-buffer-file-coding-system RET (You probably want "utf-8").