が

Topics and subjects

The differences between は and が look kind of like this:

• Bobはぼくのともだちだ。 ピザがほしい
  (Literal direct translation) Bob. (He) is my friend. Pizza want.
  Bob is my friend. (He) wants pizza.

It’s the last sentence I want you to notice. There is no topic mentioned, but there is a subject; pizza. However, it makes no sense without knowing that it’s bob who wants the pizza.
 
Generally,  は sets aside something as a topic, and subsequent sentences and subjects relate to the topic. (until changed). 

が for specificity

が is preferred over は when you have a narrow, specific target, especially when it already relates to a topic. が tends to place more emphasis on the subject
Using が puts emphasis on the subject of the sentence. 

が must be used when the words modifying the subject are specific to a particular individual or group. 

In the following sentences, が is generally preferred:

• ねこはこわい – Cats are scary (normally thought of as scary, i.e. not necessarily to me). 
• (私は)ねこがこわい – Cats are scary (to me)
• かれはじょうず – He is good (skillful)
• かれがじょうず – He is good (skillful)

 Some words, like adjectives and states, are by their nature specific:

• (私は)猫が好き – I like cat(s). (i.e. like is specific to cat; for me)
• ピアノがじょうず – Good at the piano. (“good” is specific to pianos)

You may still use は with these words only if you intend to make the subject the topic, and there is no other topic (like I). 

”が” words

Other words that will typically use が due to being specific to a thing.

• すき (liked)
• きらい (hated)
• こわい (scary)
• じょうず (skillful) 
• へた (unskillful)
• Question words