Я скучаю по тебе. (Ya skuchayu po tebye)
This translates literally as "I miss you."
Here are some other ways to say "miss you" in Russian, depending on the context:
* Мне тебя не хватает. (Mne tebya ne khvataet) - "I lack you" (more formal and literary)
* Я по тебе тоскую. (Ya po tebye toskuyu) - "I yearn for you" (more poetic and intense)
* Скучаю! (Skuchayu!) - "Missing you!" (informal and abbreviated)
The best choice will depend on your relationship with the person you're talking to and the tone you want to set.