Rub each glass rim with a lime wedge and dip the rim into coarse salt.
You can shake the cocktail in a shaker or stir it right in the glass. Combine tequila, lime juice, and agave nectar in the glass with ice and stir to combine. (Or, you can shake it it a cocktail shaker with ice and then pour into the glass.)
Top it off with grapefruit soda, give it a gentle stir and enjoy!
Nutritional note: each type of soda you will choose will have different nutritional value.