Я очень скучаю (Ya ochen' skuchayu)
Here's a breakdown:
* Я (Ya) - I
* очень (ochen') - very
* скучаю (skuchayu) - miss (present tense)
You can also say:
* Мне очень не хватает тебя (Mne ochen' ne khvataet tebya) - I miss you very much
* По тебе очень скучаю (Po tebe ochen' skuchayu) - I miss you very much (literally: "I miss you very much by you")
The choice of phrase depends on the context and whom you are talking to.