Don't worry about fastboot, that's more for your PC.
The question is do you have twrp installed. You can see by rebooting to recovery to see. If it come up with a blue ish page something like what is in the attached, then you have.
If you haven't, then you need to, and that's when you need to use your pc and fastboot mode.
Firstly download the ROM you want to install to your PC or internal memory. Then reboot your phone to fastboot and follow the instructions on the ROM download page or use the information in the attached guide. When you reboot into recovery, you will now have twrp installed. You can mount your PC to copy your ROM over or just use twrp to INSTALL, select ROM file, swipe to install, when done you may have a page that says wipe dalvik and cache, if so, just swipe again and then reboot to system and you're done. Do not format at that stage or you will wipe your phone, just wipe dalvik and cache only.
Hope this helps but please ask if not.