Add pineapple, banana, and yogurt to a blender. If using protein powder, add that as well. Add the milk, starting with ½ cup.
Blend until smooth. Add more milk if needed to thin out your smoothie.
Notes
Get yourself a good blender- I use this blender and love it. You don't need a super fancy or expensive blender, just one that will get everything blended together nicely.
If possible, always use frozen fruit for smoothies. It makes them so much creamier!
Troubleshooting: If your smoothie is too thick, add more milk (or water). If your smoothie gets too thin, I blend in a couple of ice cubes to thicken it back up.