A lot of it depends on what kind of site you are marketing; however, I would say the first step for any site is to get it indexed in Google. You can do that by quality inbound links to your site. You could focus on one of your optimized keywords, create a backlink with the keyword as the anchor text, and link it back to the optimized page of your site. Sites good for creating backlinks include ezine articles, squidoo (as long as its not spammy), contribute to forums within your niche with your link in your signature, and the list goes on and on Joy
First off, making it 100% fully functional. Many people I see want to jump straight to the marketing side of things, yet, no one can even order on their site yet. So what's the point? If you are here, next step would be to make it SEO friendly. If you aren't familiar with this, do a little bit of reading. You want to make sure you have keyword-rich URL's, proper page titles, clean code, etc. You are bound to have some things to work on here, I'd recommend getting to them early. Once you get passed this, write down your marketing plan. This doesn't have to be anything particularly difficult. Go with a plan though, I recommend it to everyone. It helps when it comes to testing things out. For example, if you want to get into article marketing, don't wear yourself out, this is a game of stamina. Get a marketing plan together with article marketing where you are only doing 1 or 2 articles per day, unless you plan on outsourcing this. Factor in link building such as directory submission, blog commenting, etc. Devote some time to getting involved in the target community. More than likely there is a forum out there for the target audience, get involved, offer something of value, gain some trust. All of these are great things, but you need to work on your plan.