"Quiero pizza porque tengo hambre."
Translation:I want pizza because I'm hungry.
Note for Spanish learners:
In the Spanish sentence, pizza should be in italics or in quotation marks since the word hasn't been adapted to the Spanish orthographic system. The plural, pizzas, shouldn't a special typography since it's a hybrid (the plural has been formed following the Spanish rules).
Shortening because to cause is definitely informal, but not considered slang by everyone anymore. You'll find lots of people who won't accept it in writing, even if they speak that way.
Hambre is hunger. I'm Spanish, you "have hunger". You can't say "estoy hambre", "I am hunger", unless you have become the living embodiment of hunger ;)
Apart from what DanD_8 said, you can also say "Estoy hambriento", but it's more common to say "Tengo hambre."