It's not important to have many followers. Being yourself is what matters.
100% free
Register as a Brander (this is what we call our influencers).
Brands can see your profile and invite you to join their promotions. For each promotion it will be clear what the brand offers you in return for your help.
Create your posts on the social media platforms you already know and love.
The Brands you work with rate your efforts and the better you do, the more brands will reach out and ask you to join their promotions.