If you have no experience rearing kits then you should not attempt to give it milk – you can do far more harm than good. In fact, you can make the kit inhale the milk into it’s lungs and kill it.
The kit should still be with it’s mother – most reputable breeders do not wean kits until at least 6 weeks old (8 in the UK) and do not sell them until they have had a good couple of weeks on their own.
They need to be capable of eating and drinking independently of their mother and should gain weight and be healthy before sale.

Edited to add – I see you have asked this question before ( ) and gotten a couple of answers – asking more than once is not going to give you the answers you want to hear.

I also note that you have joined Chins-n-Hedgies and received some excellent advice there too – please heed the advice given already.