You are going to have to do 90% of niche selection yourself. If you get spoon fed you won't ever get anywhere, and people will feed you saturated niches. Also you might end up in a niche you have no interest in, but if you are just on IG thats not a big deal.
I would take people's suggestions which will be usually - Fashion, Makeup/Beauty, Gaming, TV Shows, Hot girls, Game Hacks, Health / Fitness, Memes.
However, to find a real niche you will have to dig a bit deeper yourself, and find one that is more targeted but underneath this list.