Too many beginners don't really know what a genre is and too many wannabe's think that a genre is "limiting." So, to help, here's how genres breakdown. Sort of.
After some quick notes: First, don't make the mistake that a genre is all-encompassing. Too many beginners think that a genre has to touch on every aspect of the comic. It doesn't. Just because your mecha comic has a running joke doesn't make it a comedy; it's only a comedy if the whole comic is supposed to make you laugh, not have points at which you laugh. And drama/comedies don't exist; if the comic has a lot of drama in it, as well as comedy, just leave that part of. ABC Networks tried "dramedies" in the mid-80's; although the shows (such as "The Wonder Years" and "Doogie Howser") were popular and critically acclaimed, and are even thought fondly of today, they quickly ran from the "comedy" aspect of the concept and became half-hour dramas.
Second, don't feel that it's limiting: Genres are wide open spaces and you can always make up new genres. You can even go Chinese menu with. Have fun with it; there have been mecha westerns, medieval magical girl horrors, even futuristic magical soap operas. My personal favorite has to be SF/Action/Adventuer/Detective/Cannibalism/Independent Film/Disco/Post Apocalyptic/Greasers/Hippie/Satire; "Radioactive Dreams" (Tagline: "Just your typical action-adventure-science-fiction-musical-fantasy in the post-nuclear world") is a great movie!
Realize that genres are extremely general classifications, and that they are more for marketing than anything else. All it takes to be considered a "western" is horses, frontier, and a lot of desert. And even those are negotiable. So, without further ado, here are twenty-four genres for you to start with:
Sci-fi: Involves technology and the isues created by technology.
Fantasy: Involves magic and the issues created by magic. Usually b&w morality, but not mandatory.
Historical: Could have happened; happens before the present day. Has no magic or tech that didn't exist at that age.
Modern: The majority of the action happens in today, or around it.
Pulp: Generally, modern but exagerated. It's darker (no one is innocent and life is cheap), more violent, and heroes are more gray than white.
Comedy: The basic idea is to poke fun at conventions or just have fun. Not as easy as it sounds, and too many people try anyway...
Drama: Serious. Shakespeare/Greek Tragedy serious.
Romance: Comic where most of the challenges and drama are based on two people finding each other and looking for a long-term relationship.
Hentai: Comic where most of the challenges and drama are based on two people finding each other in weird positions and looking for a short-term relationship.
Mystery: Drama with an emphasis on solving some sort of crime.
Horror: Comic with an emphasis on unnerving or scaring the reader. Generally allegorical, and can be lovecraftian (psychological horror featuring other-dimensional beings), splatter (fear-factor depends on intense and messy physical violence), or thriller (straight psychological horror).
Action/Adventure: Generally B&W morality with an emphasis on action. There may be some philosophical musing, but the final showdown is on how survives, not who ascends.
Martial Arts: Action Adventure, but the emphasis is on martial arts.
Racing: Action/adventure based on racing.
Super-hero: Action/adventure featuring characters with psuedo-scientific abilities.
Magic Girl: Generally a girl with special abilities who has found herself under a boy's authority (Urusei Yatsura or Oh My Goddess).
Magical Girl: A girl or group of girls with magical abilities that require some sort of transformation in order to become powerful.
Shonen: "Boy-style." B&W morality with an emphasis on overcoming challenges. Ironically, as it believes that there are no useless characters, if females are present they are in a command position or occasionally take center stage, making it less sexist than you would assume.
Shoujo: "Girl-style." B&W morality with an emphasis on, ironically, combat; generally has romantic elements. Generally, has to be done well as it borders on parody.
Mecha: Specific sci-fi sub-genre that features human-controlled robotic vehicles, generally human-shaped.
Post-apocalyptic: Take world. Do a lot of damage to it. Technological comes in hi-tech/scary and low-tech/not as scary flavors. Add mutants to taste.
Western: Generally, looks like the American Southwest, with a B&W morality (or at least, evil actions have nasty consequences), and emphasis on story-telling rather than violence (although it can be highly violent, the emphasis is on story and symbolism). Technology generally limited to transport and ranged weapons.
Game: Comic with an emphasis on video/computer games; in essence, the emphasis is paralleling various game universes and not really caring about the fourth wall.
"Real Life": Comic with an emphasis on realistic reactions and either parodying real life or attempting to simulate it.