Do your homework to find a search engine that's right for your needs. You'll have to evaluate different search options, from downloading freeware to having your Web host or a third party provide your search capability.
Your Web host or solution provider may already run a site search system and be able to add your site as an additional index or directory.
If you have friends or colleagues with a spider-capable search tool on their site, find out whether they'd be willing to host your search on their server.
There are some freeware options to consider. FreeFind, Thunderstone, and whatUseek intraSearch are free and will visit your site and create an index of your pages hosted that will be hosted remotely. This means you can offer search capability for your site without having to install anything but a simple HTML file.
See CNET's Search Tool Listing for a survey of other site search tools available on the Internet.