I’m really sorry if you found this post because you were looking for a specific answer. The biggest lesson I have learnt so far on my PhD (apart from maybe that classes never start on time because everyone is always late??) is that everyone’s project is different.
For me, I started at the library. Which, if you had to pick a place to start, is probably the best bet. You’re the only person who can really decide how to start though, because it’s your PhD. It’s time to start making big decisions. You’ve done well so far, you’ve chosen a topic and a university, maybe a place to live, the supervisors who are going to help you with your project, maybe a new rucksack to help you carry all of those library books around (I’d really recommend a good rucksack, look after your back, pals). Ask yourself what inspired your project. Is there a key figure in your field whose work you need to read before you can begin? What sources will you be using for your own research? Do you need to look at specific archives?
Apparently I’m lazy because I have chosen to study contemporary celebrities in film and television, and can mostly just use the internet and the library. Lucky me! You might be different though. Your first priority is making sure you’re going to be able to access all of the resources you need to complete your project to the best of your abilities.
Trust your instincts. As you begin your research you will get a feel for what’s relevant and what isn’t. Some areas you will need to explore in depth, and other you will just need to cover for background context. The start of your PhD is the best time to explore widely, soon you will realise that even an 80 000 word thesis really isn’t big enough to cover all of the aspect of a topic that interest you and you’ll begin to narrow your scope. But for now, be excited and go immerse yourself in your research.