How do you start introducing domain driven design to your projects

So recently our project started growing and the code is pretty much all over the place when you mix admin, user interface and the overall website into one single pile which is called App namespace. I'd want to split the code into domains and then subdomains I've read how to come up with the domains and I'm somewhat good to go but there is the concept of agregates and I dont quite understand how to tackle them could someone bring some explanation for example how to combine it with the project entities and if the agregate itself should be an entity there are a lot of questions I'd love to ask a experienced developer