SEO is absolutely, certainly the backbone of any web marketing program. Without it your website will most likely remain hidden in the shadows, unseen by web users and will fail to make sales. An SEO agency can manage the search engine optimization of your website, driving traffic. The staff will have a great experience of what makes the search engines tick and will understand the best ways to implement a marketing strategy which finds the balance between obtaining visitors to the site and making sure that they are qualified leads who are likely to make purchases. Ultimately a website with no visitors and one with visitors looking for something entirely different from what the site offers are both as bad as each other.
So how do you find an SEO agency? There is no correct answer here. If you have employed a web design company to design and build the website you could ask if they have a web marketing team: many digital agencies, known as full service digital agencies have their own SEO agency team and there are many advantages to having your SEO Strategy managed by the same company which built the site in the first place.
Another way of finding your SEO agency is to collect collections from your industry and others for recommendations. There are a lot of agencies out there who will use unethical tactics which could get your site blacklisted. Others are just plain poor at their job and you'll see little return on investment.
Finally, check out Google. Search SEO agency, or digital agency or any related keyword. It's fair to assume that any good agency will feature highly in the search engine results for relevant terms. After all, if they do not then they're definitely not very good at their job, are they?