So I announce the end of my turn.
My opponent responds with a lightning bolt to my head. It resolves so I take 3.
Do I still have priority to do something else, eg to play instants that says do this when you take this much damage etc~?
In your case, you do have priority and you can cast the instant in question. However, an instant that says "When you take [some amount of] damage, [do something]" won't really do much because the damage has already been done, so to speak. However, if you had responded to the lightning bolt with your instant, then it would have worked as intended.
You ask specifically about the end of the turn and responding to damage, but you also seem to not completely understand the priority system in general. There are several points during a turn (upkeep, end of draw step, main phase, beginning of combat, attack step, block step, end of combat step, end step) when the active player (the player taking the turn) gains priority. Whenever a player has priority, they can either do something (play a spell, activate an ability) or they can pass priority. Spells and abilities don't resolve and steps don't end until every player has passed priority. Usually, people take the shortcut that if somebody doesn't say anything, they are assumed to be passing priority.
The important thing is that every player gets priority every step (as long as someone gets it) and before anything resolves (so you always get an opportunity to respond). You can read the comprehensive rules entry about timing and priority for more information.
In your specific case, when you say "I end my turn", it really means "I propose a shortcut that we skip to my end step" (there is also a comprehensive rules entry about Shortcuts and when your opponent cast the spell, they were implicitly accepting your shortcut, casting the spell during your end step, and then passing priority to you.
This answer assumes that you are playing with Competitive or Regular REL, which includes your typical Friday Night Magic. I explicitly mention tournament rules when they apply, so just know that they do not apply in a non-tournament setting.
Your opponent did not specify when he was casting Lightning Bolt, and so he is assumed to be acting during your end step because of the following tournament rule:
The game moves forward to your end step, with a Lightning Bolt on the stack, and you have priority because of yet another tournament rule:
The game continues normally from that exact point. You have a few options:
In short, the game can only progress when all players pass in succession. Your opponent decided to cast Lightning Strike in your End Step, and you get the option to respond.
No, it is so much simpler than the complex responses we give for the stack and priority. Every time you cast a spell it goes up above the table in your hand, you reveal it so the person can read the card and then imagine a small timer going off where you can choose to stop it and interact with the spell being played or "allow it."
Simplest and easiest way. When the spell is in the air (or action of any tapped ability or paid for ability other than mana) and the player is casting it, this is when the opposing player has "priority," which is really just your ability to respond. A sorcery can only be played on one of the two main phases, so the only way to respond is with mana abilities (which cannot be stopped if they are activated and is the one exception to this general rule unless stated on a card.) What the mana pays into (instant spells, abilities of creatures, enchantments, lands, -permanents or what you cast from your hand or activate through abilities of pernaments.
Here is your scenario.
"Pass turn." -you
Opponent receives "priority" because like everything in magic and responds with lightening bolt. You now have priority again, and can respond to his lightening bolt which actually would go on top of his spell (the stack) in the air and would go first) but because you moved your life counter down, it is assumed the spell resolved and the turn was over as you had moved intentions over to ending the turn.