The opening Rashi of the sedra notes that just like man was created after all the animals, so too are we given certain laws [leprosy here] after the animal details were given in the korbanos sections. But of all the ‘human’ laws, why speak about tzara’as first?
In creation, it was man who completed the creation of the world, and is thus the most important and beloved member of creation to HaShem. HaShem wants us to complete His world by doing mitzvos and serving Him. And in putting the tzara’as portion first, the message is to remember that as much afflictions as we might get, we are still the most important members of creation and are HaShem’s Children. And so, HaShem wants us back, He wants us to commit to Him – the punishments and afflictions are therefore for our good. That is why tzara’as is put first in the human laws here so we can make that parallel with the creation of the world and realise that a Father wants his sons back, no matter how many things the son does wrong.