Fitness niche is very competitive and is not easy to start making money from a fresh site.
You need to focus on delivering quality content before you sell. Build trust with your audience, offer solutions to their problems.
Instead of selling straight away, offer some free download to get those visitors on your email list.
Before you start collecting emails, you will need to set-up a series of emails that will get sent to your email list subscribers. For that you will need an auto-responder like Mailchimp (which offers a free plan up to 2000 subscribers).
You have to plan out what emails you will send to them.
As a suggestion, you can start a 7 day email series that teaches something of value to your visitors who feel compelled to join your email list.
There are lots of products here on plrdatabase.net which you can split into day-wise content.
I hope this helps you to get a basic idea about how to go set-up your blog for making money.