All depends on budget, get a thermal undersuit, I got a set for around £30 make sure that your outer layer is not too tight as air is also a good insulator. Textiles over leathers as they are lighter and easier to make waterproof.
I utterly love my Rukka kit, but that is not in everyone's budget. Layers will help keep warm and allows you to tailor how warm you need to be, but the outer layer needs to be 100% waterproof if out in the wet to stay warm as if it leaks then it allows cold in.