Prepare Glass:

Optionally, rim the edge of a margarita glass with a lime wedge and dip into salt or Tajín for a salty-spicy rim.
Mix Margarita:

In a cocktail shaker filled with ice, combine tequila, triple sec, and fresh lime juice.
Shake:

Shake well until chilled.
Add Mountain Dew:

Strain the mixture into the prepared glass over fresh ice.
Top with Mountain Dew:

Top with Mountain Dew. Adjust the amount to your preferred sweetness and tartness.
Garnish:

Garnish with a lime wedge on the rim.
Serve:

Serve immediately and enjoy your Mountain Dew Margarita!