It is best to combine 1 and 2. I recommend click here
3 and 4. I don't think you register with oscommerce, you just download the software and install it on your web space. I set up oscommerce a while ago. Can't actually remember but I think it was straight forward, just follow the instructions.
You will need to set up payment processing. Pay pal is also very easy to to join and oscommerce (i beleive) intergrates well.
5. Whole lot 30 quid!