Hi Lily, I think this is a very broad question and not related to a specific LCA software like openLCA so I don't think you can get a detailed answer for your question here. In general you would need a database that contains the necessary building materials like concrete, gravel, cement and so on. More importantly you'll need to calculate the energy consumption (natural gas, electricity...) and maintenance needs for your building during its full service life which can be tricky. Then there's the decomission phase which involves demolition, recycling, waste manegement etc.
There's plenty of materials out there if you google "building LCA", also there's a standard called EN 15804 which deals specifically with buildings. Hope I could help,