admin.py

from django.contrib import admin
from .models import Category, Contact

admin.site.register(Category)
admin.site.register(Contact)

models.py

from django.db import models
from django.utils import timezone

class Category(models.Model):
    name = models.CharField(max_length=255)

    def __str__(self):
        return self.name

class Contact(models.Model):
    name = models.CharField(max_length=255),
    surname = models.CharField(max_length=255, blank=True),
    phone = models.CharField(max_length=20),
    email = models.CharField(max_length=100, blank=True),
    created_at = models.DateTimeField(default=timezone.now),
    description = models.TextField(blank=True),
    categoty = models.ForeignKey(Category, on_delete=models.DO_NOTHING)