If you plan to purchase marijuana as a recreational user under Prop 64, the only thing you will need is valid state identification showing you are 21 and older.
If you are a medical patient, you can acquire a card by following the instructions linked below, but you are not required to do so. You would not have to pay a sales and use tax on marijuana purchases if you have this card:
You can get a state card now. See here: http://www.cdph.ca.gov/programs/MMP/Pages/default.aspx After Prop. 64 passes, you will still be able to obtain a recommendation online. You will only need to have a state ID card in order to take advantage of the sales tax discount at an adult use store. You can still use your online rec to access a dispensary now or one licensed under MCRSA starting January 2018. However, if you choose to get a state card and Prop. 64 passes, you will be able to use it at any dispensary and get the sales tax discount starting Nov. 9th!