Cut a lemon and rub edges of glasses with it. Then dip the glasses in a plate that has either sugar or salt or a combination.
Combine tequila, lemon juice, triple sec and agave syrup in cocktail shaker with ice. Shake for 20 seconds. Strain in a margarita glass and garnish with a sliced lemon.
Notes
Take the time to freshly squeeze the lemon juice. It is cheaper and just tastes better than store-bought bottles.
Use agave instead of simple syrup. It takes takes less work and adds additional flavor.
Use silver tequila when making the margarita. Gold tequila can muddy the flavor.
Used crushed ice which will make the margarita colder quicker.