You need to apply to either the City Council or the relevant Co. Council dependant on the area you are planning to buy in, the initial paperwork to apply for the scheme can take up to a month they will determine how much you are allowed according to your salaries, there is a ceiling but I'm not sure what it is, I imagine the websites of all LA's have details on them. Then if and when you find a property you like within budget it can take up to 3 months from the bid to moving in, shorter of course if the property is completely up to spec, mine was held up for about a month as it wasn't correctly vented. So short answer is, if you see something you like this weekend don't bid on it with the expectation that you'll be financing it this way unless they are willing to wait.
Why don't you go to a broker anyway and see if you can get a mortgage, while Shared Ownership can be handy it can also be a very expensive route if you don't have the means to re-finance within a few years.
Good luck!