Learn English – ny difference between being ill and sick

'Yes' can be considered the baseline - I can't think of any context in which using it would be inappropriate. (It might come across as slightly over-formal on some occasions, but more likely no-one would pay attention to it) Any official/formal speech will most likely use 'yes', as will almost all written usage.

'Yeah' is an informal equivalent to 'yes'. In casual conversation you are more likely to hear 'yeah' than 'yes' unless the speakers are making a deliberate effort to be precise with their speech (the reason that 'yes' can potentially sound overly-formal). On the other hand, a written 'yeah' will generally only be used when writing dialogue.

'Yah' would be 'yeah' spoken in a different accent. I would personally associate it either with a non-native speaker (the German accent in particular), or with native speakers who are uneducated/unsophisticated.

Personally, I wouldn't do either; I light a candle.

Kindle is a slow process, like when you're starting a fire in a fireplace. Kindling is little bits of wood or other material that you feed to the fire to get it going. Personally, it sounds weird to me if you're not talking about something like a wood fire.

Ignite is quick. If lightning hits something and sets it on fire, it was ignited, not kindled. You can ignite gas, but not kindle it.

Both of these words have metaphorical uses which similarly are different in the implied speed of the action. Someone's work might kindle interest in a subject while a controversial comment by a politician ignites debate.