Someone told me this when I first started and it really is important. New players often don't realize all the things that happen when you die and how it starts to tip the balance away from your team. Almost no enemy champion death is worth the trade. Play conservatively, don't over-reach. Don't get ahead of your minions and DON'T get into a field of enemy minions next to a tower to chase an enemy.
Things that happen when you die:
- They get a big boost of gold for your death.
- You aren't farming gold or xp so you fall behind
- Your enemy IS farming gold and xp so they get ahead
- Your partner in lane now has to handle it on his own so it limits his ability to farm
- Your enemy in lane isn't getting poked so farming is easier for them
- Your enemy in lane doesn't require 2 people to cover the lane anymore so they can wander over and gank another team mate.
- Now that they're ahead of you in levels and gear, they can farm faster and easier so they'll get farther ahead.
- They are ahead in levels and gear so they can push the lane faster.
Please people, practice not dying. If you see the green number in the upper right falling behind the red number, play conservatively. The way to improve that ratio is NOT to aggressively try to kill more enemies so the green number goes higher, that will only get you killed.