I
verb
BAD : Children should be taught that it is wrong to litter.
GOOD : Children should be taught that it is wrong to drop litter.
BAD : If someone litters the street, someone else has to clear it up.
GOOD : If someone drops litter in the street, someone else has to clear it up.
◆◆◆
People drop/leave litter (= leave things such as empty packets, cans and bottles in a public place): 'There is a $100 fine for dropping litter.'
Things litter a floor, street, area etc: 'Discarded clothing began to litter the dance floor.'
II
noun
BAD : In the cities the streets are full of litters.
GOOD : In the cities the streets are full of litter.
◆◆◆
Litter is an uncountable noun: 'Wherever you find crowds, you'll find litter.'