To create a drop down list, first we list the items we want to list in the cells:
Then click on the cell that we want to create the drop-down menu and select the Data option from the Rebon tab and select Data Validation from the Data Tools group:
In the window that opens, place the Allow value from the drop-down menu to the list, then set the Source value to the titles desired (in this example, the months of the year).
Note: You can manually enter the values for the drop-down list, and separate each item with the comma mark from the next item. Like the picture below: