You can Co-op as soon as you're finished in the tutorial area. In order to summon other people in your world, you must be in ember form. Usually people place their summoning sign around a bonefire and that's were you want to look for a co-operator. If you want to join other people's world and help them, you need to buy the white soapstone from the old merchant lady in Firelink shrine. It costs only 500 souls so it's easily obtainable. Then you want go to a bonefire and place your summoning sign by using the white soapstone and wait for someone else to summon you.