If you really want to do the EEPROM part first then start by looking at the EEPROM example sketches as suggested previously. But I don't recommend you start your project with this for a couple of reasons:
You have not decided how you will store the data – the hexadecimal characters you intend to use can be packed so that two fit into a single byte. It is also not clear if you need to store the entered number or just the index. Your post implies that the access codes for each apartment are hard coded into the program, but what happens if someone moves their access code needs to be revoked?
The other reason not to do the EEPROM part first is the EEPROM memory (unlike RAM) has a finite number of write cycles. The number is large enough so that well behaved code will not be a problem, but if you have a programming error and a loop gets out of control you can accidently exceed this.