Je manque
This literally translates to "I lack" but is used to express missing someone or something.
Here are some examples:
* Je manque mon ami. (I miss my friend.)
* Je manque ma famille. (I miss my family.)
* Je manque la France. (I miss France.)
Other ways to express missing someone:
* Tu me manques. (You miss me.)
* Il/Elle me manque. (He/She misses me.)
* J'ai envie de toi. (I want you.)
* Je pense à toi. (I think about you.)
The best choice depends on the specific context and who you are talking to.