Is it okay to start with a blog then add a membership feature after some time, or is it okay if i have the membership site first setup then add a blog afterwards?

niche is earn money online, it's saturated but here locally there's a good competition and a good chance for everyone as well

Really depends on what you want. If I was you, I'd jump straight into doing a membership site, there is no reason you can't have both, supply basic "general" knowledge in your blog with an upsell to your membership.

I recommend you reading the $2000 / Month Membership site thread that is active in the Offline Marketing section.

Personally, I think that you start with the blog and provide some pretty good content to get them hooked on your knowledge. Once you have a "readership" you can start the membership site OR you could simply give snippets of premium content and when they go to read more it would require payment.