Opening files

For opening DOS text files:

to read files:
open "file.txt" for input as #1
do
line input #1,s$ 'gets an entire line from file and puts it into s$
loop until eof(1) 'eof function reutrns -1 when end of file is reached
close

to write to files:

will erase everything in the file and start writing:
open "file.txt" for output as #1
print #1, "hello" '<prints "hello" in the begining of file
close

will modify the file adding content to the end of the file (not erasing everything):
open "file.txt" for append as #1
print #1,"hello" '<will print "hello" at the end of the file
close

if you want to modify other files that are not DOS text files then:

'GET filenum,offset,storage
dim s as string*2
open "file.bin" for binary as #1
get #1,1,s '<will get the first 2 bytes of the file
put #1,33,s'<will put 2 bytes of data into the 33rd byte in the file
close

I'm not sure how to modify random access files, 'cause I think they are stupid.

I suggest you read the QBasic help files, they probably have a better explanation. If you don't have the QBasic help files, download a proper QBasic distribution.

Key statements:
OPEN
PRINT #
WRITE #
INPUT #
LINE INPUT #
GET (file)
PUT (file)
FREEFILE
CLOSE