You have a intention to keep fast so you just get up 15. mins earlier before the azaan and you find your self najis, need to do ghusl e janaba so what do I do? Can I do suhoor in the janabat state of janabat or should I take ghusl then only I can do suhoor but till I take ghusl azaan is given what should we do in this scenario?

Ghusl has to be before imsak. Sahar is not necessary but Ghusl is necessary before imsak