There are two ways to do this, depending on which mode you're on.
If you're on singeplayer and have cheats enabled, you can type "/gamemode survival" (without quotation, including the slash) and you will be in creative.
However, if you do not have Cheats enabled, there is a tool for changing game modes - see the related links for the MinecraftForum link.
If you're in multiplayer, you can change the game mode from survival to creative in the same manner as singeplayer, the only catch is you must be an op. An op is an operator of the server, or better known as an administrator or moderator.
-Mr. Andy
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Depends, do you like to to build with unlimited materials, health, and flight or trying to fight of monsters whit whatever you can find Sometimes what I do is I make a fort in creative and switch it to survival (press esc and click on creative or survival to switch to the other)
You use it to gain experience points in creative mode, you can only get it in creative mode, but then you can switch to survival with cheats.
I would recommend you get the SinglePlayer Commands mod for Minecraft. It is very easy to install so don't worry about a difficult setup.
You need to turn cheats on and then press / and then type in gamemode c to switch to creative mode or gamemode s to switch to survival mode or gamemode a to switch to adventure mode
The 3 existing modes on minecraft are adventure, survival and creative.