cheats

[Mr. "I need help guy"]

how to enable cheats ex, infinite fuel or is it a mod?

[Geonovast]

Just use the "debug" menu.

If you're on Windows, it's alt + F12

Linux is Right Shift + F12

Don't remember what Mac is.

[Zeiss Ikon]

More generally, it's "mod"+F12.  Which key is the mod key depends on your OS.  As noted, that's Alt for Windows, right-shift for Linux, and I believe on Mac it's the Apple key (can't say for certain -- I have access to a Mac belonging to my partner, but I've never used it, much less installed KSP on it).

[Streetwind]

...And on consoles, use the "Konami Code".

(No really, it's not a joke. Only works in the new "Enhanced Edition" though.)

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions.