I had the same exact problem when I was in fifth grade. Right now I'm about to go into tenth grade, and I've long learned my lesson.
I hated math when I was your age because I didn't get anything, so I asked my dad to check over my homework every night, and help me with the problems I didn't understand.
Turning in things on time is a part of growing up. Your school is trying to teach you responsibility, which we all have to learn at some point in time, whether it's the easy way or the hard way. First of all, be determined. If you really want good grades and want to be a good student, now is the time to start. If you're a good student in fifth grade, middle school and high school will come easy. Trust me, I know

!
So write yourself a little note on a scrap piece of paper the moment you get an assignment, and put it someplace you know you'll look at or be before the next school day. Such as on your kitchen table at your breakfast table setting, or even set it on your cubby hole or locker to find in the morning. Eventually, you'll be so used to finding the notes, you won't need them anymore.
As for the fifth grade, I found that to be fun! You are the one who knows the school best, and you're one of the bigget kids there. All the little kids look up to YOU because they are scared. They know that you are somewhat the leader of the pack.
So relax, and just remember, everyone else in your grade is going through the same thing. So did everyone else in the world, including me

! Good luck!