Where to begin? As Mouse says, that is a very general question. You need to think about a lot of stuff;

1. Do you intend creating your Dungeon maps digitally or with paper & ink or a combination of the two? If digital comes into the equation, which software do you want to use? Photoshop, Gimp, Campaign Cartographer (with the Dungeon add-on) or somethng else entirely?
2. Have you played any role playing games; Dungeons & Dragons/Pathfinder, one of the many retro-clones, Runequest in any of its variants, GURPs, Savage Worlds et al? This is not necessary to create a Dungeon map but it is useful as you will learn what is required in a Dungeon map.
3. Do you want your maps to be basic and useful, artistic, or some combination of both? There's no point creating a beautiful map if it can't be used.
4. Google is your friend, Google D&D maps, Mike Schley, , Jonathon Roberts, J. Edwards, Rob Lazeretti, Adam West, Dyson Logos, Tim Hartin and pretty much anyone else that creates maps, you'll find plenty to look at. Decide on the style you want, personally, I like old-school style but many prefer the Schley style (and who can blame them, the man is a genius).
5. Check in the sub forums here, there is heaps of useful info and examples including many tutorials and objects.
6. Check out Fantastic Maps by Jonathon Roberts (aka Torstan) he, like Schley, is a legend.

Get yourself some graph paper (i.e. gridded paper) and a pencil; draw some geometric shapes (lightly) varying the number of squares in each "room", connect them with corridors and tunnels and then pretty them up however you wish. Viola, your first Dungeon.