ranked will be full of smurfs. You want to make sure you are not only confident (not overconfident) and have a very objective-centric train of thought.
Smurfs will shit on you if you are overconfident or unprepared. Also you're lvl 21, you do not have full runes and masteries and that alone already puts you at a disadvantage. That's not good.
Also, you will be matched with teammates that also have a good chance of not being good and can end up feeding freelo champs at low elo levels (fiora, yasuo, leblanc, etc) and that alone can literally throw the game. Of course the same can go for you as well, you might get lucky and get matched with a smurf on your team that will carry the unholy crap out of you and your team like Hercules, but that's not something you should be trying to immediately account for whenever you match up.
First get to 30, experiment with runes and masteries in normals and get very comfortable with AT LEAST TWO different positions usually with champs you like playing (example, you like tanky champs? then try laerning top and support, maybe even jungle if they have good CC. like mages? mid and sometimes support are good positions for mages. Make sure you know what your champ does in their team position). Also, DO NOT get thirsty, especially early om where you can die to a few hits and do not dive, you will probably get the crap outplayed out of you if they have someone competent enough to take advantage of your thirst.
Your first goal after getting comfortable with your champ and playing after hitting 30 and getting your runes and masteries all sorted out will be trying to get positioned outside of bronze, because bronze is like a whirlpool that will keep dragging you back in no matter how hard you try to swim out of it so you want to be placed outside of it from the very beginning.
