You need to do :
5->dim(L1)
You can find "dim(" in the catalog (press "2nd" then "0") and L1 by pressing "2nd" then "1".
Now you list is composed by 5 values.
Try to execute "L1" and you will have {0 0 0 0 0}
Now to put data into L1 simply do 12->L1(1)
If you execute L1 you will have {12 0 0 0 0}
There's a lot of helper to play with list, you can sort, find max, min ...
If you need a bidimensional list, look at matrix !