The most simple expressions to use would be "I like you a lot. Please go out with me (sukidesu. tsukiatte kudasai.) ", "I've liked you for a long time (zutto mae kara sukideshita.) " etc. Potential curveballs include "Are we dating now? 'Cos if we were, I'd be really happy (ima no oretachitte tsukiatten・no kana? moshi soo dato shitara, sugoku ureshiindakedo.) " or "Look, I said I hated you. But that was a lie (ore sa, anta no koto daikiraitte itta kedo, sore uso dakara.) ". As well as these lines, other possible strategies include declaring one's love using the lyrics of a song at karaoke, or, in a scene common amongst school-themed manga, someone writes how they feel in a letter then shoves it into their target's hand with the words "Um, please read this (ano kore yonde kudasai.) " before quickly making a getaway. There are lots of different methods for letting someone know you like them, but in the end it seems that it's best just to let your feelings known in a simple and straightforward way. As for the response of the person being asked out... If they feel the same, they might simply say something like "I've like you a long time too (watashi mo sukidatta.) " or, humbly, "If you're sure you want me then please go out with me (watashi de yokereba, tsukiatte kudasai.) ". If they don't dislike the person, but aren't sure if they want to enter a relationship with them right away they might say "Let me think about it (chotto kangaesasete.) " to put off replying , or take a formal first step towards dating by saying "Let's start off by being friends (otomodachi kara hajimemashou.) ". To refuse someone's advances while also sparing their feelings, often someone will use a line like "I'm sorry, there's someone else I like (gomen・nasai, hokani sukina hito ga iru.) "or "I like you, but only as a friend (tomodachi toshite nara ookei.) ".