Depends. If you want to make custom models of weapons, buildings and stuff, you will need to learn at least basics of 3D modelling and texturing. If you want to make NPCs, followers, quests, new places, dungeons, cities, islands etc, you won't have to, as CK already has all the resources used in the base game. It would be best if you would describe what exactly are your modding plans. But I have to warn you - it's better to start with simple mod, like new Follower, or custom house so you can learn the basics of CK