Here are some ways to use Ahrefs SEO tool for free:

Use Ahrefs' SEO Toolbar: Ahrefs' SEO Toolbar is a free extension for Chrome and Firefox that provides valuable SEO data about the pages and websites you visit. Its free features include on-page SEO report, redirect tracer with HTTP headers, outgoing links report with link highlighter and broken link checker, and SERP positions.

Use Ahrefs' Free SEO Tools: Ahrefs offers several free SEO tools that are perfect for those with a tight budget looking to do audits, build links, optimize content, and more. These tools include a free keyword generator, SERP checker, backlink checker, and rank checker.
Take Ahrefs' Free SEO Course: Ahrefs offers a free SEO course that teaches you how to use Ahrefs' tools, data, and SEO metrics to improve SEO for your website. The course materials are available to Ahrefs users, and no sign-up is required.

Use Ahrefs' Website Checker: Ahrefs' Website Checker is a free tool that automatically scans your website for SEO issues. It provides you with a detailed report on your website's health, including issues with broken links, redirects, and more.
Use Ahrefs' Webmaster Tools: Ahrefs' Webmaster Tools is a free account that provides you with access to some of Ahrefs' SEO tools, including the Site Explorer and Backlink Checker. You can use these tools to analyze your website's backlink profile, monitor your rankings, and research content ideas.
While these free tools are limited compared to a paid Ahrefs account, they are still immensely valuable for anyone who's just starting out in SEO