For question one see documentation here: https://docs.mendix.com/refguide/drop_down
For the second question I advise you to follow the starting course you can find here: https://gettingstarted.mendixcloud.com/index3.html
I think you'll need to be more specific:
1. Drop down is typically used for either an enumeration value or a reference selector. Enumeration being an attribute sourced by an enumeration list and a reference selector being a value from an associated entity. This would be the 1 of the 1 to many association.
2. Typically values in a datagrid are already stored in a table. What are you trying to achieve specifically?