To indicate that you are prepared to depart, the word 'READY' is sufficient to inform ATC. This word is not normally used in aviation communications in any other scenario, so no chance of misunderstanding with ATC exists. You can also extend it's use:

• Ready for departure
• Ready on reaching - during taxi as you approach the holding point.

You must NOT use the phrase 'ready for take-off'. Take-off shall only be used in response to a take-off clearance issued by ATC.

ICAO Doc 4444
12.3.4.10 Phraseologies for use on and in the vicinity of the aerodrome
12.3.4.10 PREPARATION FOR TAKE-OFF

c) ARE YOU READY [FOR DEPARTURE]?;
d) ARE YOU READY FOR IMMEDIATE DEPARTURE?;
*e) READY;

* Denotes pilot transmission.