Q:

Django: how can I a create categories field/dropdown menu? 

This is a very simple question, how can I create dropdown field in Django with only specific categories(something similar to the countries dropdown, but not with countries). 

A:

With the choice attribute of a Field if it is for fixed values.  Or with a ForeignKey field if the Categories should be created dynamicly. 
For the ForeignKey field you would do the following:
from django.db import models

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

    def __str__(self):
        return self.name

class Item(models. Model):
    category = models. ForeignKey(Category)
    # ...

