Writing 'I Love You' in Japanese
The most common way to say 'I love you' is 'ai shiteru.' Ai is the word for love and has a commonly used kanji to write it. Shiteru is the present continuous tense of the verb suru (to do). The first phrase is how one would write 'ai shiteiru' in its entirety including the kanji for love, or ai. Then, the kanji for love, or ai, is by itself.
Saying 'Ai Shiteru' to someone who is not ready to hear it or not used to hearing it may be scary, so do not say this to just anyone. It is also not a friendly form of love that you would say to a best friend, pet, or someone you care about. This is a phrase you would use in moments of strong feeling.
Notice that both of the kanji used when referring to love contain the kanji for heart, or 'kokoro.'
Another word used for love is kou. This word/kanji in combination with the word hito for person becomes koibito, or a lover; however it is not often used to say the actual expression 'I love you.' The first kanji is the kanji for 'ko' or love. It is not usually used to say 'I love you' directly but is used in combination with the kanji for hito to make the word, koibito or lover as seen in the second example.