How to stop procrastinating on homework?

I'm 13 and in 8th grade. In the beginning of the year i was doing pretty good.. i didn't procrastinate too much and i was an honor student. Now i'm doing TERRIBLE! when i was in 6th and seventh grade, i would always do an assignment the day it's given so i'll get better grades and won't have as much homework later in the week. I was so concerned about my grades at the time i didn't even hang out with friends or leave the house! For some reason, in the middle of this year, it's not that i stopped caring about my grades, i'm really stressed out about them actually! It's just that when i get home, I never feel like doing homework. I just come home, eat, listen to music, play on the computer and start homework at like 7 or 8. then at times when i really don't feel like doing my work, i just say i'll do it in the morning which i usually do. What can i do to stop procrastinating? this has really lowered my grades :( thanks!