So, by default you won’t have a .bashrc file so just navigate your to your home directory by typing:
cd ~
create or edit the .bashrc with vim (or whatever editor you are comfortable with):
vim .bashrc
Here is the line I had to add to mine (I am running a 64 bit OS so if you aren’t don’t copy this exactly)
alias notepad="/c/Program\ Files\ \(x86\)/Notepad++/notepad++.exe"
If your copy of windows is 32 bit then it should look like this
alias notepad="/c/Program\ Files/Notepad++/notepad++.exe"
Finally, close and reopen your terminal/bash (or, as noted, run source ~/.bashrc), and voila!