The first rule when it comes to naming a variable is that the name you give it should be meaningful; the second rule is that you use camel case. Let's take a common example from games and declare a variable to store a player's health:
public int health = 100;
If you find yourself declaring a variable like this, alarm bells should be going off in your head. Whose health? Is it storing the maximum or minimum value? What other code will be affected when this value changes? These are all questions that should be easily answered by a meaningful variable name; you don't want to find yourself confused by your code in a week or a month.
With that said, let's try to make this a bit better using a camel case name:
public int maxHealth = 100;
Remember, camel casing starts the variable's name with a lowercase letter, then capitalizes the first letter in each word thereafter. It also makes a clear distinction between variable and class names, which...