Not bad for your debut hunt! Mine was a 20 mm sea urchin spine (broken) from the upper chalk, not nearly as impressive as yours!
Firstly you must soak it in tap water, changing it several times over several days to remove any salt. This will also soften any clay matrix which you can remove with a stiffish brush/a pointed object / knife / whatever you can find. then dry it out slowly and finally bruch off any loose material and coat it with something that will keep air & moisture out because of the pyrite. As to what you use in this case doesn't matter too much. nail varnish, hairspray, PVA, varnish, Paraloid - you name it, theyve all been use by someone to delay pyrite decay!
sorry, as for genus I'm rubbish at IDing ammonites!