You can absolutely do SEO by yourself.  You know your content. You know the keywords you rank for. If you do use an agency, they are going to be asking these questions anyway. From there, you need to determine the level of effort you are personally willing to invest in SEO.  Doing it well is a combination of both technical skills, content writing and content marketing.  There are many great companies that can help you rank higher.  Remember though, that ranking higher typically takes time and isn’t going to happen overnight. It doesn’t hurt to get some help, especially for the first part in creating an SEO audit and roadmap.