You need good shelving. There is shelving you can buy from an office supply story (plasic and mesh are most popular) but the best and cheapest is shelving made at home. See what others are do and ask the advice of your senior advisors.
The best locker shelving is made at home. Measure the width and length of your locker; then cut three pieces. Two can be the sides at whatever height you want; the third piece will be the shelf. Do this and you will have inexpensive and very sturdy locker shelving!
Arrange your books by subject. If you can just grab a set of books/notebooks for each class, you will be quick and you will not forget. If you have to search through your locker, remember what books to bring and then find them, you might often be late and come to class "forgetting" a lot.
Have some kind of holders on your locker door. Put a pen, your assignment book, whatever you can and need handy.