Trying to find my best 2-3 roles in League

Greetings, fellow summoners, I am in need of pro assistance. I need to know how best to find and fill about three roles in a five member team. Up to this point, I've been playing primarily tanks and failing at it. My actions in matches have been described as trying to be a burst mage/assassin type with a tanky character. This was about three years ago since I last played. Hoping to start fresh, I want to find out which two-three niches I can best fill.

My play style in almost any game I've played (shooters, MMORPGs, etc) is to dash/sneak into a fight, burst in guns/spells blazing, and end up dying after clearing - hopefully - a lot of the enemy forces; I also have no concept of backing out of a tight spot till it already has killed me. This has not worked for me in League, but that is because I summon tanks (Garen and Darius mostly). I thought of trying over with mages and relearning how to play, but others have recommended burst assassins. I'd like others opinions on what course to take. If burst assassin, which champions should I favor? If mage, is Swain still any good?

This and any other help to a summoner in need of relearning how to play this awesome game would be helpful.