This will depend on what is written in your lease. You will normally have to give the housing association some time to find a buyer for your share. If they ‘nominate’ a buyer to you then you must sell you share at a value set by an independent valuer. You may also be required to pay the housing association a fee for finding you a buyer.
If the housing association do not nominate a buyer, you would normally list your home with a local estate agent. Any buyer you find will still need to be approved by the housing association.
You also have the option to buy the remaining shares and sell the property outright at the same time.