Replace spaces in post title with underscores

Hi!
I’d like to replace the spaces (and is possible the special characters) in the post title, on the post page, with underscores ‘_’
I’ve tried myself but my php knowledge is very limited and couldn’t made it. I suppose the simple way is to modify the <?php the_title();?> in the single page file, that’s what I’ve tried.
Can anybody help me on this?
Thank you!