Try this:
1. power off -- all the way off (hold down the power on button for about 6 seconds).
2. press and HOLD down these keys:
Command-OPTION-R
...and now press the power on button.
KEEP HOLDING THOSE KEYS DOWN
3. When you get asked for your wifi password, let go of the keys and enter it.
4. Now, you should see "a spinning globe", which indicates that it's trying to boot into INTERNET recovery. It takes a while, so just wait it out.
5. Do you get to the internet utilities screen?
If you do...
1. Don't run the installer yet
2. Open disk utility
3. IMPORTANT: Go to the "view" menu and choose "show all devices"
4. Look at "the list on the left". The topmost line is the physical drive inside (we need to erase it)
5. Click the top line to select the drive, then click "erase"
6. Erase it to "Mac OS extended, journaling enabled, GUID partition format"
7. When the erase is done, quit disk utility and open the OS installer
8. Start clicking through the installer. The Mac will reboot one or more times, and the screen will go dark for a minute or more with no indication of activity. Be patient.
9. When done, you should see the initial setup screen (choose your language).
10. Start clicking through and get yourself set up.