Quick answer
Я скуча́ю по тебе́
ya skucháyu po tebé — по + dative — one of the few places it appears
The options
Я скуча́ю по тебе́.I miss you. — по + dative
Я скуча́ю по вам.I miss you. — polite or plural
Мне тебя́ не хвата́ет.I miss you. — literally “you are lacking to me”
Скуча́ю!Missing you! — short, in a message
Когда́ уви́димся?When will we see each other?
Two different sentences
Скуча́ть по takes the dative: по тебе́, по до́му, по Москве́. The other version flips it: you go into the dative and the missing person into the genitive — мне тебя́ не хвата́ет. Both are common; the second is a shade more literary.
What you hear back
Я то́же скуча́ю.I miss you too.
И я по тебе́.Me too.
Ско́ро уви́димся.We’ll see each other soon.
Typical mistakes
Я скуча́ю тебя́.
Я скуча́ю по тебе́.
The verb needs по plus the dative.
Я скуча́ю по тебя́.
Я скуча́ю по тебе́.
по takes the dative, not the accusative.
Check yourself
Which is correct?
по + dative. (The third is a regional form you may hear in the south.)