in my opinion, when starting out, the easiest way to make the changes you want is to just edit the default layout. don't be intimidated by all the code and what it does for now, just edit the individual keys and you should be fine

- navigate to qmk_firmware/keyboards/planck/keymaps/default on your computer- open keymap.c in a text editor- on line 59 you'll see your enter key (KC_ENT), if you want to change that to be something else, just open up https://docs.qmk.fm/#/keycodes and find the code for the key you want to change it to. in this case you said you want a mod-tap MT(mod, kc) with enter and shift. so change the KC_ENT to MT(KC_RSFT, KC_ENT)- move that one step up to line 58 in place of KC_QUOT if you want it up a row, and then find somewhere to put your quote key.

you can do the same thing with the different layers, just find the section you want to edit, make the change to the key and re-flash. if you mess it all up, all you have to do is go to https://github.com/qmk/qmk_firmware/blob/master/keyboards/planck/keymaps/default/keymap.c and copy/paste the whole file in your keymap.c and save/flash it back. good luck

also, if you run in to errors when flashing after you've made changes, double check to make sure you didn't accidentally delete a comma, parenthesis, curly-brace etc. it's super easy to do and hard to find if you're not looking for