If you have knowledge about the SEO and search engines guidelines especially google then you can do your own. But I recommend you to hire an SEO agency or Freelancer because if your domain is penalized by Google then you can’t recover your domain. There is only a 5% chance to recover the domain. If your domain is penalized by Google then you have to buy a new domain name, and again have to do website development work. If you don’t perform SEO activity properly you will not get any positive results then ultimately you have to hire a professional SEO person.