If you're using Twine for the First Time:
Once you've installed Twine, you may want to take a look at the official How to Create Your First Story page on the wiki. There are also some very good videos covering Twine from the basics through to extending Twine 2 with javascript. Vegetarian Zombie's Intro to Twine 2.0 is a good place to start. Both of these resources are for Twine 2, rather than version 1.42. I recommend getting started with Twine 2. There are some tradeoffs between Twine 2 and 1.4.2, but that's a whole 'nother topic.
What if I want to add pictures to my Twine?
To make a long story short, this is the number one reason to use Twine 1.4.2. You can use externally-hosted images in Twine 2, or use a base64 encoder to add images to your game, but because Twine 2 is .html-based, it can't store images as binaries. Twine 1.4.2 complies a .html website from code ("twee") so you can add images directly to Twine 1.4.2 and see them in your project.
Why not just use Twine 1.4.2, then?
Twine 2 is a clear improvement in terms of ease of use, and as long as you're not learning bad habits, it's easier and faster to get started with. Also, a Twine game is fundamentally textual: images can add something, but you may want to stop and ask whether your story is improved by illustration. Poorly chosen illustrations in a Twine can mar it, similar to the effect of opening up a novel only to find that someone had scribbled on the pages.
Twine 2 has some distinct strengths: unlike Twine 1, it fully supports CSS and javascript, making it much more versatile and extensible for high-end use. It's easier to get into CSS than javascript, and there are all sorts of interesting visual effects you can achieve in CSS in just a few lines and without any need for "graphics" in the conventional sense.
What if I want RPG mechanics, fancy text, or other bells and whistles?
There are some great resources out there that show you how to up your Twine game without having to learn javascript. In addition to the official forums, the Twine Cookbook explains how to do thing and is full of cut-and-paste examples: leave it open in a browser tab while you work. To make effective use of the Twine Cookbook, you'll need to know what story format you are using: the default for Twine 2 is "Harlowe."
Glorious Trainswrecks is full of advanced Twine use, especially javascript (along with other experimental, unpolished game design stuff). Beyond that, you can find all sorts of things with a search or two. Experiment, explore, and have fun!