If there was only one single piece of advice we could give you for building and launching your new website, it would be this:
Take nothing for granted.
Just because something should look, or function, a certain way, that doesn’t mean it’s always going to. Just because my site looks a million dollars when viewed on Mozilla Firefox and Google Chrome, that doesn’t mean Internet Explorer is going to load it the same way. Nor we should assume that just because we find it easy to log on to my site and use all its features, doesn’t mean my customers are going to enjoy the same way. Perhaps they naturally navigate the site in a way we hadn’t thought of, or come up stuck at a point we assumed would be pretty straight forward.