According to Google,

Deciding to hire an SEO is a big decision that can potentially improve your site and save time, but you can also risk damage to your site and reputation. Make sure to research the potential advantages as well as the damage that an irresponsible SEO can do to your site. Many SEOs and other agencies and consultants provide useful services for website owners, including:

• Review of your site content or structure
• Technical advice on website development: for example, hosting, redirects, error pages, use of JavaScript
• Content development
• Management of online business development campaigns
• Keyword research
• SEO training
• Expertise in specific markets and geographies.

Keep in mind that the Google search results page includes organic search results and often paid advertisement (denoted as “Ads” or “Sponsored”) as well. Advertising with Google won’t have any effect on your site’s presence in our search results. Google never accepts money to include or rank sites in our search results, and it costs nothing to appear in our organic search results.

Please do watch this video where Google’s Maile Ohye [Developer Programs Tech Lead] shares her advice for hiring an SEO (Search Engine Optimizer) consultant or agency to improve the searcher experience on your website: