Giving you the right answer won't be enough. I want you to understand why "I miss you" is "mi manchi" and not "ti manco" and what the difference is between the two, so that you will never forget.
In English the idiom is:
someone (subject, who misses) misses somebody/something (object, what/who is missing or missed)
"I miss you" follows this pattern exactly. But if we want to say the same thing in Italian, the pattern is:
qualcuno/qualcosa (subject, what/who is missing or missed) manca a qualcuno (object, who misses)
NB that subject and object are inverted here! We also add a preposition "a", meaning "to". So, following that pattern:
I miss you
becomes
you miss to I
you miss to me (you don't say "to I" in English, you say "to me")
and translated into italian:
tu manchi a me
Now "tu", which is the subject of the sentence in Italian and the object in English, can be understood, as we usually do for the subject Italian and you get:
(tu) manchi a me
manchi a me
Unlike English where "to me" remains "to me", "a me" usually is abbreviated into "mi" and shifted before the verb, but after the subject, and you get:
(tu) mi manchi
You can also add the subject back and say:
tu mi manchi
but we use "tu" rarely, only if we want to stress who the missing person is.
On the contrary, if you want to say "You miss me":
you miss me
you miss I (me becomes I before the inversion)
I miss to you (invert subject and object and add the to preposition)
Io manco a te (translate)
Io ti manco ("a te" becomes ti and shifts before the verb)
ti manco (the subject is understood)
Remember, ti means "to you", "a te".
And if you want to say: He missed her? Same game!
He misses her
She misses to him
lei manca a lui
lei gli manca
gli manca
So, "he misses her" is "gli manca". Remember gli=to him. Now "she misses him":
she misses him
he misses to her
lui manca a lei
lui le manca (a lei -> le)
le manca
Remember, in "le manca", le=to her and the subject (lui) is understood.
When there is ambiguity you may want to express the subject:
lei le manca = She misses her
lui le manca = She misses him
Marco le manca = She misses Marco
Anna gli manca = He misses Anna
Btw, this is how I teach Italian to my students, by comparing its structure with English. If you want to take a lesson with me, you will learn Italian this way, effortlessly!