You cannot just build a web site and expect customers to find you. You do have to advertise. If you are only targeting local customers advertise in the local newspaper(s), join the Chamber of Commerce, send out a press release and use SEO to target geographical searches.

If you are seeking global customers then you need to spend the time and money on good SEO. If your products or service is fairly generic then using Google AdWord, PPC or other paid services may be necessary.

Having good incoming links also help. Remember not all links are created equal. Avoid link farms like the plague.