If you’re building website traffic, you may not be eligible for network campaigns, but you can probably find work that pays be being an affiliate for brands you like. You can find them through affiliate networks like ShareASale or the following

If you’re a fitness blogger interested in ambassador programs for networking, writing opportunities, paid campaigns, and sponsorship, you might check out the links for applications to see what fits your blog and social media presence.

Communities:
FabFitFun Ambassadors
FitFluential Ambassadors
Genuine Ambassadors
Girls Gone Sporty Ambassadors
Idea Inspired Bloggers
I’m Fit Possible
Popsugar Select
Sweat Pink Ambassadors
For Two Fitness

If you’re looking to join networks in order to be considered for brand campaigns, consider:

Clever Girls
Collective Bias
Global Influence Network
Linqia
Massive Sway
Mom Spark Media
One2One
Pollinate Media
She Speaks
Activate
IZEA

Some bloggers use ad networks to make money and you can find a long list of ad networks at WAH Adventures.

I hope that helps you get started monetizing your blog.