What is the shortcut key to start a new line in the same cell?

Alt + Enter: Start a new line in the same cell

Some important shortcut key:
Ctrl + Enter: fills the selected cell range with the current entry.
Shift + Enter: completes a cell entry and selects the cell above.
Ctrl+Home: moves to the beginning of a worksheet.
Ctrl+Shift+Home: extends the selection of cells to the beginning of the worksheet.
Source: Microsoft support