铅笔(I wan't to pratice step by step in chinese,I'm starting with a really basic on(to wan't)

你想要來顆蘋果嗎?(Do you want an apple?)
我想要一支铅笔(I want a pencil)
我想要一顆苹果( I want an apple

铅笔(I wan't to pratice step by step in chinese,I'm starting with a really basic on(to wan't)

这儿有铅笔吗?(Do you wan't a apple?)
我要铅笔(I wan't a pencil)
我要苹果( I wan't a apple

 i just can't understand why you write want as "wan't" and why don't you write "an apple"???~~~~(>_<)~~~~