If niche is highly competitive then which keywords i should select. In this case, do i need to select only avg competition advertiser? https://adwords.google.com/select/KeywordToolExternal and keyword is mobile wallpapers
if your keyword is mobile wallpaper, i will suggest you work on highly competitive kwds but if you want the results soon, u can also select some related keywords as well
but i read high competitive keywords can not be used for getting quick rank. I always ignore related keywords, why i should choice them, any reason.