If I have ball in hand behind the headstring, and there is a ball in the kitchen, can I shoot that ball or can I only shoot balls beyond the headstring?
I suppose it would be helpful to have a video of all the different rule situations as well as the standard ways of replying to the situations. Until that is available, it's a good idea to read through the rules at least once. Once is probably not enough, since some of the rules don't make much sense until you encounter the situation. Here are the rules:
https://wpapool.com/rules-of-play/
To answer your question directly.... If you have ball in hand as the result of a scratch by your opponent, you must drive the cue ball over the line before it can hit any ball behind the line.
If you have ball in hand because the cue ball was in the rack area when it was time for a new rack, and the single loose ball is behind the line, you must place the cue ball on the head spot and you may play the loose ball.
(And if that last situation happens with the object ball blocking the head spot, the cue ball goes on the center spot, between the side pockets, but it is likely you will never see this happen.)