What questions should I ask about my shares in a new business?

I have been offered a job in a new internet business as a sales person (UK) The company is starting out and the website is being developed but not launched yet. The owner has said I will have some kind of share of the business, around 10 or 12% if I help grow the business. I was telling a friend and she asked me how the shares part was going to work. For example how long do I need to work for the company to get my shares. Is it performance related and that kind of thing. All this is very new for me because I have never worked for a new company before. I would really like some advice about the questions I should ask in my next meeting with the owner so I don't get any surprises later.