When building a website, the goal is to make it as accessible to both people and the bots. While this may seem like a contradiction, it is not. Today the internet bots are basically simulated searchers and they look for the same content that human web surfers look for. In order to please the bots, we must make our website desirable to people as well as easy to navigate. In order to make a site easy to navigate, we must include a sitemap.
You should first create a sitemap for you visitors. The bots can wait. In order to operate a successful website you must first please your customers. It is no different from any other business. To do this you will need to create an HTML sitemap. You should do this with the same software that you used to create the rest of your website. There is no reason to differ from the style of your site for a sitemap. If you are lazy, there are sitemap creators available for free on the internet that will provide you with a generic HTML sitemap. This will make your website look half hearted so I don't recommend using them.
Next it is time to please the bots. To do this you will need to create an XML sitemap. God forbid someone program the bots to look for HTML but what are you going to do? This is where you can use an XML sitemap generator to do the work for you. I say this because your visitors will never see this sitemap unless you provide a link to it. You will need to submit this to Google and other search engines for it to have an effect, but once you do, you won't have to again. The search engines will look for that specific webpage in order to crawl your website completely, so you can update without submitting it again.
Creating and submitting a sitemap is essential to good SEO. You can get by without it, but why? Go out, create a sitemap, and prosper.