To start, you might want to check out these threads:
http://smnetwork.org/forum/index.php/topic,2517.0.htmlor
http://smnetwork.org/forum/index.php/topic,2397.0.htmlor
http://smnetwork.org/forum/index.php/topic,2655.0.htmlThe short answer to your question is that you don't always need to call a warning - in fact, you usually don't. Warnings are just that - a warning that a cue sequence is coming up (say, if a cue requires more prep time than normal). Standbys are basically telling your operators to put their finger on the button and wait for your go to push it. Standbys are always necessary; warnings are not.
I usually call my standbys about a half-page before the cue, depending on how complicated it is: "Standby light cue 104 and sound cue H."
You should get back responses from your operators: "Lights." "Sound."
Then when the cue comes, I say, "Lights 104 and Sound H GO."
Sometimes you might need to be a little more verbose in your standbys: "Stand by light cue 104 and sound cue H. I will call lights first and then sound."
Everyone calls cues in their own way - as long as you don't hesitate ("Lights...um...go? I guess?") and speak clearly you'll be fine!