searching for helpfull tips

So its been 3 weeks now since i started playing lol and i just hit lvl 30. im playing jungle ( kindred ) and mid ( zed and kassadin ). the problem is that i am useless. when i play zed for example i always play extremely defensive,because im scared to die and feed. this makes me useless and even tho i know what my problem is i cant fix it, because when i try to play aggressive i simply die and are the feeder of the team who lost the game. so im asking for maybe some tips on my problem in order to become at least a little bit usefull