File management is fairly simple under Microsoft BASIC 80. In general, you
have to follow the same rules as CP/M (since this is what it's running under):
- All filenames are allowed an 8-character base name with a 3-character
  extension (For BASIC, generally .BAS)
- Filenames MUST be capitalized to work properly with all CP/M programs. Why
  this isn't done automatically, I will never know.

You can SAVE and LOAD files using their respective commands. Remember that
filenames are strings, and as such have to be in quotes (""):

  To save a file:
  SAVE "FILENAME.BAS"

  To load a file:
  LOAD "FILENAME.BAS"

If you don't append a file extension, it will be assumed to be ".BAS", so you
could, in theory and in practice, replace the above commands with SAVE
"FILENAME" and LOAD "FILENAME", respectively.

In order to delete a file, use the KILL command.

In order to clear the current buffer and start a new program, use the NEW
command.

The CLEAR command will clear all currently declared variables out of memory.

===
It seems to me that this is generally done automatically in BASIC under CP/M,
but BASIC also has a "RESET" command, which will write the directory
information out to the disk. This is important to do before switching disks,
according to the manual, but I assume this is under Disk BASIC.