How would I make an installer?

You shouldn't need one. If your program requires multiple files, place them all in the same folder. Then at the program upload menu, select that folder instead of a file within it. The entire folder will be uploaded as one.

But I'm making an eshop type program ;-;

To be hosted where? If it's for the Nintendo 3DS e-shop, you won't be able to post it, since your program has a dependency on SmileBASIC. If it's for someone's virtual/emulated "OS", ask that person what format you need to provide.

Im making a program that you can download more programs on!!!

make it print the public code to the app/game when the "purchase" is complete. then, use CHKFILE"the file's name"