This is going back some for me & things might have changed but I think you need to learn WML (wireless markup language). It's extremely similar to HTML with 'pages' arranged as 'cards' in a 'deck'. It's a form of XML (extensible markup language).
Yes you can edit your files in notepad no prob.
In terms of hosting, you need a WAP-enabled (wireless application protocol) hosting service.
I actually tried to run a few WAP pages from my usual host - and it worked. I had to fiddle about with the http headers as I remember, I think I wrote a perl script to pump out my WML with the correct headers. Or come to think of it, it might have been a case of editing my .htaccess to get the server to recognise the correct mime types. It worked for a few weeks, I could access the pages I'd created on my mobile 'phone - but then it mysteriously packed in. Maybe the host caught on & changed something; they were upgrading their servers at the time though so possibly that had something to do with it.
If you want to use images you need to use WBMP images (wireless bitmap) - there are a few converters floating around on Google that'll let you convert a BMP or JPEG or GIF. (with pretty severe loss of quality).
There are hosting services around that will automatically detect the type of handheld at the other end of the line, as they vary in what image formats they'll accept, and serve the correct image format automatically, not sure how this is done, whether it's an 'on the fly' type thing or whether you have to supply a selection of different image formats for the server to choose from.
I believe this service is available from tagtag.com but when I last looked their site was down.. hmm
Anyway, as I say, I was playing around with this maybe a year, 2 years ago even, so things will have moved on a bit I guess.
At any rate, it's not frightening & a good google session should turn up the bones you need, good luck
Here's a link to some info:
http://www.w3schools.com/wap/wap_intro.asp