You can use whatever milk you like: kefir milk (which is what I use), nut milk, coconut milk, cow's milk, rice milk, etc. which will determine if this is dairy free or nut free.
It is important to use frozen berries and/or ice cubes which will help to emulsify the ingredients, making it particularly smooth and delicious. If you are using fresh berries, then you will definitely need to use the ice to make it thicker.
To ensure a great consistency, blend for the full minute to allow the frozen fruit and/or ice cubes to break down fully and emulsify into the mix. This will give you a beautiful thick smoothie. If you like it to be a bit thinner, then add a little more cold water after you have blended and mix for another 10 seconds.
To the base mix add whatever supplements you like … generally I will add 1 tsp Camu Camu powder and 1 tsp Spirulina. Coconut oil is terrific too and sometimes I include a tablespoon. BUT, as I make my own vanilla bean ice-cream ( see here ), I will sometimes be a bit naughty and add a scoop of that too ... it all depends on what I am looking for at the time ... that is the lovely thing about making your own smoothie, you can add what you like and know exactly what is in it.