ska = someone has decided. kommer att = it will just happen.
SwedishLevel A2
ska is for a future someone has decided: Vi ska resa i juli. kommer att predicts something nobody decides: Det kommer att bli kallt.
ska is followed directly by the verb: Jag ska köpa en bil. kommer needs att: Det kommer att ta tid. In relaxed speech Swedes often drop it (det kommer ta tid), but writing keeps it.
Examples
Vi ska äta middag klockan sju.
We're having dinner at seven.
Ska jag hjälpa dig?
Shall I help you?
Oroa dig inte, det kommer att ordna sig.
Don't worry, it'll work out.
Alla kommer att sakna dig.
Everyone's going to miss you.
Han ska bli läkare.
He's going to be a doctor.
Det kommer att ta tid.
It's going to take time.
Common mistake
ska never takes att: Jag ska köpa en bil. An offer is a decision, so it takes ska: Ska jag hjälpa dig? (Kommer jag att hjälpa dig? asks for a prediction.)