Damoh

I understand ir is the infinitive of va. However what I don't get is why it sometimes has 'a' before it and sometimes not. Quiero ir a casa - I want to go home Tengo que ir a casa - I have to go home Voy a ir a casa - I'm going to go home. This last one seems like it's saying "I'm going to to go home" to me. Voy = I'm going, a = to, ir = to go... Can someone please explain this to me?