Looking for high street Japanese brands like freak store beams Cecil mc bee, jeanasis , Gu etc
La Foret Harajuku is the place you should go! It's an iconic shopping mall existing from a long time ago. I used to shop there when I was younger. Most of the shops are for younger generations from teen to twenties.
There is also Omotesando Hills but they are not really focused on local Japanese brands. I think you can find more Japanese brands in La Foret Harajuku.
Also walking along Cat Street in Harajuku might allow you to encounter nice local shops.