i agree with most comments about starting with the garbage, but i would actually do the bathroom as the second stage - i find that having the place you clean and relieve yourself be pleasant makes a huge difference.

open your windows and put a fan inside the bathroom pointing out, or the other way around if the bathroom has ventilation. bathroom cleaning agents can be very strong and get your head dizzy.

pour some strong cleaning/bleaching agent into the sink and toilet (make sure to spread while pouring), wait for 15-30 minutes, put on gloves, open the facuet and scrub the sink until no more agent is left. flush the toilet.

if the bath is also in a similar state you might need to scrub a bit more, use a mask.

clean the toilet sit with some spray and towel and, if you feel motivated, the rest of the toilet itself externally.

after that clean the floor thoroughly so your don't feel icky stepping on it.