I am familiar with java but have only just started learning to mod minecraft with forge. I followed a youtube tutorial to create custom items and blocks, but now want to spawn an entity such as a creeper in the world, next to me. However, despite looking at multiple threads, I am still not sure how to do this. Please can I have some advice!

you need to create a new instance of the Entity class (in your case Creeper),
then use Level#addFreshEntity to add the Entity to the World,
before you add the Entity to the World you can modify it in the way you want