You've got a lot to learn about the game if you want to compete in ranked. It's much more competitive, and there is less room for error than in normals. I recommend spending some more time learning the nuanced strategies of the game, as well as more about each role and how they win the game.
Zac for example, is considered a tank, it is his job to soak up damage for his team while catching and disrupting enemies to make them easier to kill. he does not function well as a damage dealer.
When you look at champions in the game client it should show you which ones are considered harder and which ones are considered easier. I recommend playing only the easy ones for a while because that leaves you with more of your attention to focus on learning strategies for the overall game.
Some easy champions to use while you learn about the game:
Top:
Garen,
Nasus,
Gangplank
Jungle:
Malphite,
Amumu,
Master Yi,
Warwick
Mid:
Annie,
Veigar
Bot:
Ashe,
Tristana
Support:
Janna,
Alistar
You can get some of these champions for free by following Riot on various media platorms and then contacting Riot Support here:
http://support.riotgames.com/
Tristana - Liking Riot games on facebook
Alistar - Following Riot games on Youtube
Garen - Following Riot games on Twitter
To learn more about strategies and other nuanced techniques and mechanics that high level players use, I recommend watching guides by youtubers that make a living teaching about league of legends. Here are a few:
Jeremy Gaming Curios:______ Phylol:__________ Stonewall008: (jungle)
https://www.youtube.com/watch?v=fCdmLmO3rEY&list=PLpnvP9blMxw5sFlTP4czIAQkse35MTjaN https://www.youtube.com/user/ThePhylol/playlists https://www.youtube.com/user/stonewall008
Dong Huap:
https://www.youtube.com/channel/UCID1M0bzAxTChjFO8oEyjyw
Gbay99:__________ foxdroplol:__________ ScrapComputer:
https://www.youtube.com/user/gbay99 https://www.youtube.com/user/foxdropLoL https://www.youtube.com/user/ScrapComputer/playlists