Lots of web designers are great at creating amazing websites, which look great and function perfectly. A lot of them are not so hot on the SEO side of things! It is a good idea to have a plan for SEO in place prior to building your new site, as this allows your designer to have some insider knowledge about the best ways to set up your pages so SEO is a little easier.
One of the main things to think about is the likely headings you will want on your site. It is important that each page has one heading tag listed as H1. This is the main heading of the page and the most important for SEO purposes. Next, you need a selection of supporting headings, listed as H2 which are similar to, but not the same as your H1.
The content is something you can often work on after your site is mainly built, however knowing which pages are going to be based on which topic is a good idea for ease of writing.