Programming :: Using .sh Script To Replace Text?

i'm trying to script a lil piece to execute in terminal so when i type something like:

scriptname textfile

the script searches the text file, replaces charecters of my choosing with charecters of my choosing

"sed" im told is the way to go, and i got summat like this going

Code:

#!/bin.sh
bash -c "sed -i.backup -e /s/char/newchar/"

(if need be, i think i can just add another line of "-e /s/char/newchar/" if i need to target more charecters or words as needed.) the issue with the above code. . . how do i get it to target whatever text file follows the command? without having to manually designate sed to it each time? e.g. in commanding:

scriptname textfile

how do i get the script to target textfile^^^?