There is much interplay between HTML/CSS and JS. The important things to learn are:
The DOM (Document Object Model). When you manipulate HTML with JS, you need to understand the DOM. It defines the structure of the page.
CSS. CSS is what you use to define the 'look' of HTML. From simple things, like font size and colour, to complicated things, like how elements flow on the page.
JS, like any programming language, can get very complicated, but you can do a lot with a little. Work on the basics - defining variables, the different data types, functions and basic logic.
For a development environment, I would recommend the following:
A 'good' browser (like Firefox or Chrome). Internet Explorer is ... fine, but you're better off learning with browsers that make a half-decent attempt at adhering to standards. IE is a dog of a thing, they just do what they want and you always need 'special' JS, it's very painful to get things working well in those 3 major browsers, and that's not counting Safari and Opera.
A nice web developer plugin. Firebug for Firefox is great, it gives you a JS console and debugging tools. Chrome has a lot of this stuff built in. Learn to love the JS console! Learn to right click and 'inspect element'. For what it's worth, IE has a developer console too.
A good text editor. Notepad is not a good text editor! Get your hands on something that supports syntax highlighting and gives you a bit of a hand with formatting. I quite like
Sublime Text, but there are literally thousands of others. Vim and Emacs are the old stalwarts but they take some learning.
Above all, have fun, JS is a fun language to learn, you can put things together quickly and get feedback on how it works quickly - you don't have to compile your program etc ... Ask questions, find some good websites like HTML5Rocks, Mozilla Developer Network, Google Developer Something, there's A LOT out there for aspiring web developers. :)
