If you want to have a salt rim on your glass, pour some salt on a small plate then rub the edge of the glass with either a slice of grapefruit or lime. Dip the top of the glass in salt and carefully turn over so you don't knock the salt off.
Add the tequila, grapefruit juice and lime juice to the glass (traditionally hi-ball but I have used a shorter rocks-style glass here). Add some simple syrup (a little more/less to taste), stir to mix then add a few cubes of ice. Top up the glass with sparkling water/club soda and serve.
Video
Notes
As mentioned above, this is often made with grapefruit soda rather than grapefruit juice + soda water. I like the fresh grapefruit version that is slightly less sweet, but why not try both and see which you prefer (or make based on what you have).