You never use 's' for homework since it can be both singular or plural. So, you would always say, "I am now preparing to do my homework." *NOTE: This sentence sounds a little formal. You might want to say "I'm doing my homework" or "I'm preparing to do my homework" if it were a more casual conversation.

"Homework" is an uncountable noun. You can do pages of homework, or hours of homework, or a little/lot of homework: the homework refers to the group of tasks, not the individual assignments. It is similar to other uncountable nouns like "information" or "training" or "advice."