Abilites do very little damage?

Hi before i get started i must say im completly new to league of legends and this post might seem a little noobish or cringe worthy but in any game you must learn to be able to play. So whenever i fight someone on league of legends my abilities do very little damage and it takes about 20 hits to actually kill someone as for them they kill me in 3 hits. Iv tried many champions but still they all do very little damage. People in game call me dipshit since i dont kill anyone and often feed the enemy. Anyone know why this happens?
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