"I have to prepare for winter."
Translation:Jag måste förbereda mig för vintern.
It doesn't sound OK with just 'vinter' in Swedish. Definiteness for seasons doesn't work the same way in Swedish & English.
I think it might be that vintern in this case is seen as something previously known to both speaker and listener, since we do know that winter is coming. We know what winter we're talking about: the next one.