Making maps for gaming can be a time-intensive task. You can draw them by hand, or there are various software tools available for making professional looking maps relatively quickly. Some resources, both for map-making software and for tips on drawing game maps:

• Hexographer - old school roleplaying style mapping software.
• Campaign Cartographer - a long-standing game mapping program.
• The Cartographers Guild - a site devoted to providing resources for game mapping.
• Map-Making in Games - Google+ community for game mappers.
• Observations of the Fox - blog by Michael Wenman, full of game mapping goodies.

Transcript

GM: Here's the map of the planet Bespin.
C-3PO: What are we looking at? There's nothing on either side.
C-3PO: Uh... Is it snow again?
GM: I promised you it wouldn't have one land type.
Han: It's a gas giant!
GM: Bingo!
Leia: You didn't have much time to do the map, did you?
GM: Go on, explore! Anywhere you like!
Han: We go to this dot.
GM: What dot? Lemme see—
C-3PO: That huge city floating in the clouds!
GM: City?
Chewbacca: Well spotted, old chap!
R2-D2: Let me in-speck-t that.
Leia: Oh lord, stop, please.
Chewbacca: A full stop?
R2-D2: Yeah, your point?
Chewbacca: This kind of thing happens periodically.