This is one of the most common phrases you will use and come across, especially if you are dating a Korean significant other. If you watch a lot of Korean dramas or listen to a lot of Korean songs, you will hear this A LOT.
My wife and I still use this a lot ;p
Since missing someone implies closeness, the most common form you will be using is 보고 싶어 which is the casual/informal form. This form drops the “요”. The phrase is a combination of the verb 보다 (to see) and the grammatical form to want (~고 싶다). We will be starting a grammar series soon so we will go more in depth about this form at some point in the future.