Before you start creating files, it is good to learn how to do simple memory management with your BASIC program. This involves archiving, unarchiving, and deleting variables. For now, let's archive the variable "A". (Be sure to unarchive it before you're done!). The Archive and Unarchive commands are located in the memory menu. Press [2nd][+][5] to get Archive, and [2nd][+][6] to get Unarchive. DelVar (which deletes the variable) can only be found in the Catalog. Press [2nd][0][D], and find the DelVar command. So, let's make a program that stores a value to a, archives it, unarchives it, then deletes it.
:0->A
:Archive A
:Unarchive A
:DelVar A
And that's it! Now, with this new knowledge, let's go on to files.