Yep, 'Bau jetzt!' or 'Baue jetzt' would be 'Build now'. It's not necessarily confusing, many words in English and German are both nouns and verbs depending on usage...
Bau is in the accusative case here and for 'der' words, that changes to 'den'. And don't worry, all this will slowly come and become second nature. But it does mean you have to learn the der/die/das and plural with each noun. It sounds a pain, but in the long run it really helps.