For the where; check through the descriptions on the various boards to decide which is a best fit. If you do make a mistake here, the mods can move your thread to the one they feel is a better fit.

For when; at the time you have something to contribute to at least one other forum member, or have some question or inquiry to get clarity on.
It helps to use the forum search option to confirm if there is previous discssioons on that topic, that would guide what content you include, or if you go ahead to make the post at all.