In Japanese, ‘I love you’ is
Aishite-ru (in a casual way)
But if you say ‘Aishite-ru’ at the beginning of your relationship or the first time you tell your partner about your love, it might sound like ‘too heavy’ or may surprise them.
If you want to tell them that you are in love with them, you can just say
Suki desu. (literally ‘I like you’)
Anata ga suki desu. (‘I like you’, ‘anata’ means ‘you’)
note; It is better to replace ‘Anata’ with her/his name or nickname, because ‘anata(meaning you)’ sounds very polite and even bossy sometimes.