Tom Collins 

To make 1 tall drink

1 ounce fresh lemon juice
3 ounces gin
1 ½ teaspoons superfine (bar) sugar
2 ice cubes
6 ounces cold club soda

1 slice orange (optional)
1 maraschino cherry (optional)

A Tom Collins glass*

Place the lemon juice, gin and sugar in the Tom Collins glass and stir with a bar spoon to dissolve the sugar.  Add the ice cubes, fill the glass with cold soda, and stir briefly.  Garnish the drink, if you like, with an orange slice and / or a cherry.

Can use some lime juice in addition to lemon or just lime.  Your preference.

Rum is the liquor most frequently substituted for gin — then called a rum Collins .  Applejack, bourbon, brandy, blended whiskey, Scotch or vodka can be also substituted. (Pick your poison!)  With bourbon or blended whiskey, the drink is called a John Collins.