Your Media Kit is the equivalent of a CV as an Influencer/Content Creator/Blogger.
Your Media Kit is supposed to be a representation of who you are as a brand . Anyone should be able to understand who you are and what you are about before they even speak to you. From the day you decide to take your online presence seriously and monetize it, you have to create your Media Kit.
Before you think of reaching out to any brand for a partnership this is the one thing you need to have and it should be well presented. You can find the details of what should be in it here.