How do I know when to help my teammates?
That depends, really. I'll separate my answer into sections.
Mid game - Late game
You should always help each other. The only reasons you shouldn't help are :
You are low on health and (the enemy has ways to dive to you OR you don't have long range abilities)
If you can't fight without putting yourself at risk, don't bother. 1 for 1 trades are generally not worth it in solo queue unless you're a support trading for an enemy ADC or Mid.
Case examples :
- If you're Nidalee and have mana, you can stay back, heal teammates, spear your enemies, but avoid going in in cougar form unless you're sure you'll eliminate the enemy with your cougar Q and avoid damage afterwards.
- If you're Ahri, you're generally a close up assassin, which is not good when you're low health. You can throw your Q and E if they're on cooldown while running away, but don't bother with your short range W.
You know your team will lose the clash even if you help
In this case, communication is the key factor. If you truly think you can't win and you have to wait for another one of your allies before you engage, tell your teammates.
Early game
The only ones probably worth expanding on are the Jungler and Mid roles. ADC and supports generally keep bot until they need to rotate when they push the tower.
Jungler
This really depends on what kind of jungler you are. Some junglers are red-buff dependent when ganking, so you have to consider your buffs as well. Generally, if your champion doesn't have a natural CC, red buff is crucial for the slow.
I wouldn't put a "how to gank" or "when to gank" part here. There are a lot of detailed YouTube tutorials for that.
To answer generally, you should gank when the opportunity presents itself. Ganking boosts yourself AND your allies, and early game is when you want your allies to have that advantage.
Damaging the enemy laner with a gank or burning their flash is a SUCCESSFUL gank. A gank doesn't need to produce a kill to be successful. If you give your laner the chance to farm freely because you made the enemy recall, that's a win.
Mid
I recommend this VERY DETAILED YouTube tutorial by ScrapComputer for Mid laning. It gives you a lot of insight on when to FARM, KILL, or ROAM.
TL;DR, HELP when you can. By winning a clash, even if a support gets the kill, you can take advantage of that and get an objective, and further snowball your whole team.
If there'a jungler v jungler fight happening at near baron at early game, as a Top laner or Mid laner, go ahead and help your jungler especially when your lane is pushed or you have an advantage at lane (meaning, when you leave your lane, you aren't leaving your enemy laner to get a free push or get a big advantage for you)
Don't care about being called a KSer. You are playing to win. Not playing to deny your allies of their "rightfully owned" kills. By securing the kill, you are driving your team to victory.
Protip : Mute toxic players. Press Tab and click the chat bubble button. They do more harm than good. You should still communicate via pings though. :)
If you think you can do so without getting either of you killed, and without losing major objectives (a baron, two towers, etc.) you should go help. If someone complains about killstealing, they're doing it wrong. If you're playing support, try not to get the last hit on an otherwise GUARANTEED kill. In most other cases, go ahead and kill the enemy champions. Your team gets more gold for an assist + kill than just a kill, and League of Legends is a team game.
Also, if you watch professional play, you'll notice that if a kill is anything less than certain, no one will try to "not ks." Anyways, ks stands for "kill secured" :)
Always help your teammate unless you know it will just end up in 2 of you being killed instead of 1. In LoL KSing does not exist. Your team nets more gold in the end the more people involved in a kill so it's ALWAYS worthwhile to help in a kill. Sure if you are the support and they are a carry you can try to let them finish, but it's better to play safer and take the kill yourself since enemies can often flash out to survive or use some other sort of mobility to get out.
Now like the other guy said in his response, your teammate is saying you are KSing, then say 'Well thanks, I'm glad I can help Kill Secure the enemy!'.
Apeasement
Apologize for kill-stealing or kill-securing. Always apologize. This alleviates any unwanted tension caused by the indecent. Even if it was 100% your kill and they just came up and tried to take it, just apologize anyways. It will do wonders to prevent toxicity. That being said some players just want to be toxic, in these cases just mute them.
Support
Only take the kill if you have to. Assists for the ADC are better than nothing, so take the kill if it means they'll escape. However, the ADC is always going to claim that you KS'd and that they had the kill, so what do you do? Apeasement, Just apologize and do what you must. Avoid taking/securing kills unless you absolutely have to.
Jungler:
If you're ahead it is good to allow others to get the kills to help snowball their lane. Even if they suck, it will (hopefully) help. If you're behind it is considered acceptable to take the kill; however, I personally would still highly consider allowing the laner to get the kill. Never-the-less it is still good use appeasement, to apologize even if you feel you're justified taking the kill.
TL;DR:
Always apologize for taking a kill, even if you're justified in taking it. The common response to "Crap, I'm sorry I took that." is usually "It's okay."
Take the kill if you need it, otherwise leave it to others. If you get a kill on accident or someone complains about you getting the kill, just apologize. It'll help your team's cohesion, increasing your chances of winning.