Hi there.
I'm sorry to hear you're feeling like this. As much as a hate to sound like Mum the first port of call here may ideally be you're GP. Tiredness all time can be the result of a huge range of lifestyle or medical factors, and while a GP may not be able to assist in solving you're tiredness right away, they might be able to rule out some of the more common causes.
For what it's worth when I was tired all the time it was due to chronic sinusitis (only found this out after I graduated), and after two years I had surgery that massively reduced my level of fatigue. I wish and will never stop wishing I didn't identify this earlier. However everyone is different and it would depend on the nature of your tiredness, a doctor should be able to work with you to identify the cause of the fatigue and help with the management of it. Sometimes it can be useful just to air the problems, and it's great you're looking to address this six months before you start, not six months after.
PhDs can be stressful experiences by there very nature so it would be best to go into it with any medical concerns not niggling on your mind, or at least in a managable state. Maybe talk to friend who knows you best, is your situation unique to yourself? or how do other people on your course feel?
This advice is free so feel free to bin it. I'm very fairly tired writing this, but hope it makes some sense.