Need some advice..

I need some advice on baby clothing.. This is my first baby and I'm unsure on what size is best to get for a newborn baby. I've been buying 0-3 months so far but I've seen that shops do 'newborn' size or 'up to 1 month' size. I'm not sure what size is best to get for a newborn baby. Can anyone advise me please? Thankyou.