I want to download an apk file to my sd card so I can take that sd card and put it in my nook to get google play? I honestly don't know what I'm doing and I have an older model of the nook color. I only have my phone so I don't even know if this is possible
You need to use either Your smartphone or the card reader. Insert appropriate for Your nook (apparently it is a tablet according to what Google told me. Have never encountered it before) SD/memory card in the phone/card reader and connect it to Your PC/laptop.
After downloading the needed .apk file on Your computer, copy it into a folder of the SD/memory card (for example "E:\Downloads" or any other name it will be given by the system) and disconnect the card reader/phone from comp. via safe way (most probably nothing will happen if You will simply plug it out, but just in case).
Insert the memory card (if it was in your phone in the first place - take it out beforehand. Don't forget to switch off devices when inserting/unmounting memory cards) into your nook device and switch it on. Go for the file browser (aka File Manager and so on), find the folder you have put .apk file before (Downloads folder for example) and open it via touching it's name. The system will suggest to install .apk file and demand for Your approval of the action.
That should be it, pardon if too much text/too detailed explanation.