First off, the most extensive English language Guilty Gear resource on the net is Dustloop. The forums have some good introductory threads for getting into the game as well as character specific subforums. The Guilty Gear section on the Dustloop wiki is also regularly updated, and has some more in-depth info on system mechanics as well as complete frame data.
I linked this before in an earlier post, but the guys at Hitconfirm made an excellent video tutorial series for Accent Core, available in both English and Portuguese! You can see the very extensive Part 1 here and the shorter Part 2 here. UltraChenTV also did an episode on Guilty Gear the day before the XBL release, going over basic gameflow and a quick character breakdown. It has been archived on their Youtube channel in four parts. Basics: Part 1 2 Characters: 1 2.
If you are fluent in Japanese, the GGXX BBS is the main discussion center for the JP community. It doesn’t have much in the way of introductory material, but the character threads are detailed and have a lot of good information. On the other hand, the JP GGXX Wiki has more information than you would know what to do with. It has detailed explanations of the system mechanics(with pictures!) and great breakdowns on every character. There are also pages dedicated to helping new players pick characters, with discussions on every character’s weaknesses/strengths and ease of use.
On the subject of picking characters, Kugler and Turbovec of Brokentier wrote a very concise character guide designed to highlight every character’s main gameplan along with their strengths and weaknesses. In addition, my friend Brett has created a character select guide which displays each character’s ease of use and range. He’s also done a fairly lengthy video breakdown of each character. It is geared towards intermediate level players who are already familiar with the game but are unsure of character selection. You can also look through the highlights section of his video archives if you are having trouble wading through the original 5 hour long video.
Since the playerbase for Guilty Gear is a lot more limited compared to the major fighting games of today, it can be difficult to get matchup experience. You might only be able to play against 7-8 different characters on a regular basis in your local scene! Thus, it’s very helpful to watch match videos to get an idea of how the matchups are as well as learn more about your own character. And this is no better time than to plug my own site! As you can see, the Guilty Gear section on the sidebar has footage of only top Japanese players, organized by year. The Accent Core footage stretches from 2007 all the way to 2012, so there’s guaranteed to be plenty of reference material for every character.
Finally, if you feel like doing some match video searching by yourself, then it’s very important to know what the right tags are. On Nicovideo, ggxx is the tag associated with Accent Core and Plus R videos, and on Youtube, ggxx is pretty common as well although some people use ggac and ggxxac. Searching for top players can be difficult, because you have to know how their name is written. And even on Youtube, it can be annoying because most names have multiple ways to be romanized! But have no fear, because my Notable Japanese Players page has lists of all the top players for every character along with the multiple ways their names can be written. The Dustloop thread for Japanese Q&A is also great and includes the single kanji abbreviations for GG character names that are commonly used on the BBS.
I hope this post has been helpful! If I’ve missed any resources, feel free to suggest some in comments.