Whether you pay someone else to design and maintain your site really depends on what it's for. I'd say if it's for a business, look at it the same way you'd look at business cards - would you design them yourself or get a professional to do it?
If you do it yourself, you'll need to learn how to use something like FrontPage or Dreamweaver to produce the HTML. You can download an evaluation copy of Dreamweaver from www.macromedia.com and have a play around with it. You can do a lot without knowing how to write HTML, but eventually you will need to venture into the code which may look a bit scary at first but actually it is pretty simple to learn with the aid of a book. If the site is going to contain more than just text, you'll also need software to produce and manipulate images. I use Fireworks (another Macromedia product), but there are lots of graphics packages out there (Photoshop, Paintshop Pro etc etc).
If you're not so hot on design, you can buy ready made templates on the web into which you can slot your content. Some of them are quite good (www.projectseven.com has some very cool ones). I believe FrontPage comes with themed templates built in.
If you have the time, there's no reason why you can't do it yourself. If not, it would be best to get a professional to do it for you. You could pay a pro to do the design and then maintain it yourself thereafter. For a decent design I'd expect to pay between 200 and 500 quid. You will have to pay for web hosting, unless you use one of the free ones which force you to display adverts on your pages - I don't know how much that would cost precisely, but would guess at around £10 a month, quite possibly less. Your web hosting service gives you FTP access to the web server, which allows you to download and upload files to update the content yourself.
As i said at the beginning, think of it as you would think about stationery and printed publicity for the business. You will get a more professional result if you pay a professional to do it, but if you have the time and a bit of design ability, you could probably do it yourself to save money. It's a false economy though if the end result looks amateurish and puts prospective customers off! If you decide to use a professional, make sure you see examples of their work, and choose someone whose work you really like.
HTH