How do I get Excel to automatically add dollar signs?

If this question is in regard to formatting, select the cells and click the $ icon in Home > Number. This can be done even when the cells are blank, and if they are later populated with a number, the dollar sign will appear automatically.

If the question is in regard to cell references (absolute, relative, or mixed), press F4 to quickly toggle through $ placement options:

1. Press once to fix rows and columns.
2. Press twice to fix rows but not columns.
3. Press three times to fix columns but not rows.
4. Press four times to fix neither rows nor columns.

For the Mac equivalent, try Command + T.