In the end you will just use Photoshop, Flash (if you prefer flash designs), and a text editor so it's best to just start off using those applications.
For graphic design you should start off by getting used to Photoshop. When I started googled for a lot of tutorials and visited websites such as good-tutorials.com, n-sane.net, depthcore.com, shiver7.com, zymic.com, and found more links to other websites on those sites. Time and practice will really be the key in learning how to create nice designs.
Once you're a descent designer you can learn XHTML and CSS. This is crucial if you'd like to bring your designs to life. If you just plan on making ad banners, posters, logos, and so on then this isn't necessary. However, if you do plan on creating interactive websites I suggest you master XHTML and CSS. Those are very simple languages to learn.
There's a lot to learn so go google around and have fun!