It might depend what you were signing up for. In a general sense, what turns you on means things that you like or enjoy, while what turns you off are things you dislike. If, for example, you were signing up for a dating site, things that turn you on in a partner might be long hair, or athletic ability. Things that turn you off could be glasses or big feet.

Now everyone wants to know what you're signing up for. Ha ha. Usually, these terms are used to discuss sexual attraction, but not always. Of the two, "turn off" is more likely to be used in other contexts, but "turn on" almost always refers to sexual interest.

What you find attractive and what don't :)

but, 'find out' means 'discover' what? why can't I use such verb whereas, I just want to tell that I discovered something ?

"I found them when I was signing up for something." You don't use the phrasal verb here.