The only difference between basic HTML and this, is the generation of dynamic content without the need of writing codes each time you update your site. I'm sure, you'll get used to it in a few days or so, it's not THAT complicated
You can write your HTML codes when you're adding content, if you like to. Just go to "global configuration" on the main admin screen and change the editor parameter to "No WYSIWYG editor", then save the configuration. Next time you try to add an article or story, you'll see an empty box, waiting for you to add the article with HTML tags. (I'm not sure if Javascript codes would work this way, a more experienced mamber could reply to this better.)
You do have CSS files, just in your template's (theme) folder. Check the folder and you'll find a style sheet there, which defines the look and feel of your site.
Modules are elements to display something on your pages. There are some very useful ready-to-use modules that come with your default installation, like "Who's Online", "Popular", "Syndicate" or menu modules. They go to the server side, pull some useful stuff and bring them onto your page to display. You shouldn't worry about them, just use

You can also download some 3rd party modules from Mambo sites and install them according to your needs later. Also, Mambo gives you the chance to design your custom modules. When in "site modules" section on admin screen, just click on "New" icon on the upper side and create your own custom modules, if you need. You can even select an RSS feed and arrange the module to display its contents (like, for example, "World News Headlines" from Reuters.) Or you can display a static image and an announcement with the help of a module. Maybe you would like to show an ad with a custom code this way (like affiliate ads). Go ahead and feel free to edit some ready-made modules parameters too, according to your needs or preferences.
When you're finished with a module design, you have to choose on which pages should this module appear and on exactly which part of the pages. Mambo is very powerful and flexible on this: There are defined places on the basic page structure: Left, right, top, bottom, user1, user2, etc. You'll soon learn these positions for modules and feel very comfortable when placing them, believe me

On module design page, you'll see the dropdown list to select the exact position for your module. On the right side, you'll see the page list to add your module. Do you want the module to appear on every page? Just select "all". Or maybe you want it to be only on the main page. Then select "main". Or if you want it on both main page and search page: Then you push the CTRL button and click on these page names.
Components are more powerful and complicated things. They can serve to do some certain tasks or to make life easier

You want to create a new poll, for example: On the admin screen, go to the top menu ->components and select "Polls". Now click "New" icon and design your new poll. Not finished yet

Go to site modules and select "Polls". Decide where the poll box should appear and on which pages. Then click on "published" box and save it. (Some components work together with corresponding modules: You can arrange the parameters on components and then display it by modules.)
You can easily arrange the order of modules by clicking on "up" and "down" icons on "Site modules" screen. (Same arrangement can be done for content items too, when you're on "content items" list page.)
Mambo uses a database to store all the required things: Content, users, module list, parameters and many more... This makes maintaining the site easier and functions more powerful. Once you decide the look and feel by selecting a template, and define the modules you want to use, everything becomes simple and comfortably fast. You input the content you want by using the "content managers" on the admin page and as soon as you entered them, your site is updated.
You can decide what type of content you'll use. The basic things don't change: You need a content section and some content categories under each section. For example, you want to publish some short stories on your site. Go to the section manager and create a new section called, for example, "Fiction". Then, if you plan to publish different types of stories, go to the categories manager, select the new section you just created on the dropdown list ("Fiction") and create some categories like "Science-fiction", "Thriller", "Romance"... When you want to add a new thriller story to your site, just select content->fiction and click "New". While inputting the stuff, choose the category "Thriller" from the dropdown list.
Maybe you would also like a solid link to your newly created section. Go to menu manager, select "main menu" and add a menu item called (like in our example) "Fiction". Choose "List - Content section" and then, "Fiction". When you saved it (don't forget to select "published" option) a new menu item will appear on your main menu called "Fiction". Click on it, you'll see the list of categories you created: Science-fiction, Thriller, Romance... Select a category and all the items entered under this category will be listed. Click one of the items and read it
You may also want the newly entered items to appear on the frontpage. Just select "show on frontpage" box on content item screen, and the new items will appear with their headlines and intro paragraphs on the frontpage, with a link just below them which reads "continue.." If a visitor would like to read the rest, he/she can click on this link. When entering the articles, news or other content items, use the double text boxes to define the intro part and the rest.
Not everything on a web site need to be dynamic. You'll probably also need some static pages, like "About our company" or "My Biography", or "Privacy Policy" or... so on... You can create some static pages with Mambo, using the "Static content Manager". Just click "New" and design your page. If you want to, you can also add a link to it on one of your menus.
Plain easy, isn't it?

You'll love Mambo, believe me. I used half a dozen CMS software, and Mambo definitely is the best, the most flexible and most fun system.