SEO Yoast is what I have used forever and it is great for helping you set up a lot of key factors. You can also just go into the backend of wordpress and set whatever paragraphs up with those particular H1-H3 tags-

In wordpress, when you create a page/post, you get, at the top, right above where you can write, a row of buttons that you can click to make changes to what you create... One of them, opens a second row of buttons.

One of those buttons, left, is for doing what you want. Normally, it says Paragraph. But, click on it, and you can select all kinds of H's.

Yoast plugins you can use for your wordpress website.

Ah, DABK posted the exact same thing I had here.

Usually the post title is the h1. Well, at least if the authors of your theme knew what they were doing.

Hello,

I'm using the All-In-One SEO Pack plugin and it has been serving me well so far.

In the post editor on visual mode there'll be a drop down Heading Menu in the toolbar on top for the selection of h1, h2, h3, etc, right up to h6. Make sure you turn on the "Kitchen Sink" for this toolbar to appear. You just need to highlight the part of the text that you'd like to make the selection for those.

In addition when you scroll to the bottom, there are fields for you to fill in for the title, description, keywords, tags, etc all designed in the name of SEO optimization. A user-friendly and effective plugin in my opinion.

Good luck!

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Thanks, Everyone.

Yoast & All-in-one WP SEO plugins is best option to implement on-page SEO in your site.