Hi Wendy!
You can use any Sha2 library (You can find many on Github - for example).
Simply place the code in the public.js directory and send every field that you want to encrypt through the function.
Make sure to send the data both from frontend (to the database) and from the backend (to the bank’s servers) through the function in order to encrypt it.
If you decide that this is beyond your capabilities I’d suggest you to check out the WixArena where you can hire a Wix Pro to help you build your site.
Doron.